Multiple frequency processing to minimize manufacturing variability of high aspect ratio micro through-vias

ABSTRACT

A method for forming a through-via in a laminated substrate by laser drilling the through-via in a laminated substrate from a top exposed surface of the substrate to a bottom exposed surface of the substrate using a plurality of laser pulses that are trepanned in a first predetermined pattern. Each pulse trepanned in the first predetermined pattern has a first energy density per pulse. Then, the through-via is laser drilled using a plurality of laser pulses that are trepanned in a second predetermined pattern. Each pulse trepanned in the second predetermined pattern has a second energy density per pulse that is greater than the first energy density per pulse. The second predetermined pattern is within the first predetermined pattern.

FIELD OF THE INVENTION

The present invention relates to the field of integrated circuitpackaging, methods of manufacturing integrated circuit packaginginterconnections, and more particularly, to methods and apparatuses forforming vias in a multilayered substrate having alternating dielectricand conductive layers. The invention further relates to methods formaking via interconnections.

BACKGROUND OF THE INVENTION

Interconnection and packaging related issues are among the main factorsthat determine not only the number of circuits that can be integrated ona chip, but also the performance of the chip. These issues have gainedin importance as advances in chip design have led to reduced featuresizes of transistors and enlarged chip dimensions. Industry has come torealize that merely having a fast chip will not result in a fast system;it must also be supported by an equally fast and reliable packaging.

Essentially, packaging supplies the chip with signals and power, andperforms other functions such as heat removal, physical support andprotection from the environment. Another important function is simply toredistribute the tightly packed I/Os off the chip to the I/Os of aprinted wiring board.

An example of a package-chip system is the "flip-chip" integratedcircuit mounted on an area array organic package. Flip-chip mountingentails placing solder bumps on a die or chip, flipping the chip over,aligning the chip with the contact pads on a substrate, and re-flowingthe solder balls in a furnace to establish bonding between the chip andthe substrate. This method is advantageous in certain applicationsbecause the contact pads are distributed over the entire chip surfacerather than being confined to the periphery as in wire bonding and mosttape-automated bonding (TAB) techniques. As a result, the maximum numberof I/O and power/ground terminals available can be increased, and signaland power/ground interconnections can be more efficiently routed on thechips.

With flip-chip packaging, thermal expansions due to mismatches betweenthe semiconductor chip and the substrate can cause strains at the bumps,and thus, could lead to failure. Regardless of which packaging techniqueis employed, material issues such as the aforementioned thermallyinduced strain causes a chip package designer to select and matchmaterials with great care.

In the manufacture of integrated circuit packaging, the chip packagedesigner attempts to obtain ever greater wiring densities while at thesame time forming interconnections between adjacent layers that providereliable circuits with as little inductance and resistance as possible.Thus, the formation of high quality via holes, or vias, that are usedfor interconnections, is an important aspect of forming high qualityinterconnections.

It has been known to use lasers to form vias in multilayered ceramicpackages or substrates. For example, U.S. Pat. No. 5,378,313 to Pacediscloses a process for manufacturing a multilayer hybrid for a ceramicmultichip module (MCM-C) device that uses a metallic conductive patternlayer formed on an inorganic insulating layer. Vias having a diameter ofbetween 25125 pm are formed by laser drilling through an inorganicinsulating layer for making electrical connections between conductivepattern layers. According to Pace, the overall thickness of theinorganic insulating layers should be less than 50 μm, preferably lessthan 40 μm, and more preferably less than 30 μm.

It has been known to form blind-vias using lasers operatingpredominately in a scanning mode using excimer gas lasers in the 193nanometer (nm), 248 nm, and 308 nm ranges. The beam is scanned over ametal mask and then focused to a spot through apertures formed in themask. However, the mask tends to absorb the laser energy, thusgenerating heat distortion, forming oxides and/or redepositing ablatedmaterial onto the surface of the mask.

The current trend in integrated circuit packaging technology is shiftingfrom the ceramic substrate-based interconnection circuit devices toorganic substrate-based interconnection circuit devices for single chipmodules (SCMs) and multi-chip modules (MCMs) because the organicsubstrate-based devices are less expensive to process and fabricate.However, the lasers known for via formation in the past are notgenerally suitable for use with organic-based multilayer packages.

The MCM-C devices are typically formed by multilayer conductive patternsthat are combined together in a co-fired monolithic structure. Eachlayer of an MCM-C device is formed from a green ceramic tape having aconductive pattern printed on the green tape. Vias for interconnectingthe different layers are punched, or laser drilled, through a green tapelayer. The individual green tape layers are then laminated together andco-fired to form the monolithic structure. MCM-C devices suffer fromlamination size variations caused by shrinkage when fired.

When relatively thinner conventional organic substrate-basedinterconnection circuit devices are attached to an integrated circuitdie, the thinner structures of the devices flex and bend more readilythan the thicker ceramic substrate devices because of differences in thecoefficients of thermal expansion (CTE) between the materials used inthe organic substrate devices and the integrated circuit die or chip,and because of mechanical stresses that occur when the interconnectiondevices and the chips are attached. Relatively thicker conventionalorganic-substrate-based interconnection devices do not experience thesame degree of flexure as the relatively thinner conventional devicesprimarily because of the differences in relative stiffnesses. That is,the flexural or bending modulus of an interconnection device increasesproportionally to the thickness the device cubed. So, an interconnectiondevice that is twice as thick as another interconnection device made ofthe same substrate has a flexural modulus that is eight times greater.

A need exists for an integrated circuit package, and methods of makingsuch a package, made of organic-based-substrate materials that havesmall via diameters and high via aspect ratios, and that provide highconductor routing density. Additionally, there is a need for aninterconnection device formed from an organic dielectric substratematerial that has a coefficient of thermal expansion (CTE) that matchesthe CTE of a printed circuit board and the CTE of an integrated circuitchip to which the interconnection device is bonded so that the bendingeffects caused by CTE mismatch are minimized.

SUMMARY OF THE INVENTION

The present invention provides an interconnection circuit device havingvias and a method for making vias in an interconnection circuit device.The vias formed by the present invention have smaller entrancediameters, exit diameters, greater aspect ratios and lower average viaresistances than vias formed by conventional techniques. Additionally,the present invention provides an improved high-volume manufacturingyield over conventional approaches because a high degree ofmanufacturing repeatability is ensured by the present invention. Forexample, vias produced according to the present invention have lowentrance and exit diameter variances, low average via resistances, andcorrespondingly low via resistance variances.

The present invention provides a method for laser drilling blind-vias ina laminated substrate, where the blind-vias have via entrances of 75 μmor less and aspect ratios of 1:1 and greater. According to theinvention, the laminated substrate has a first conductive layer, adielectric layer formed on the first conductive layer, and an outersecond conductive layer formed on the dielectric layer. The secondconductive layer includes a preformed aperture at each location where ablind-via is to be formed. The dielectric layer is laser drilled throughto the first conductive layer using a plurality of laser pulses that areapplied through the preformed apertures in the outer second conductivelayer. Each laser pulse applied to the dielectric layer has an energydensity per pulse that is greater than an ablation threshold of thedielectric layer, but is less than an ablation threshold of the firstconductive layer.

When the laser emits at a wavelength of 355 nm, the energy per pulseapplied to the dielectric material is between 0.5 J/cm² and 11 J/cm²,inclusive, for a pulse width that is 100 nanoseconds (ns) or less. Whenthe dielectric layer is made of an ePTFE matrix containing a mixedcyanate ester-epoxy adhesive, the energy density per pulse applied tothe dielectric layer is preferably between 0.5 J/cm² and 11 J/cm²,inclusive. When the ePTFE-based dielectric layer includes afiller-adhesive mixture, the energy density per pulse applied to thedielectric layer is preferably nominally 7 J/cm². When the laser emitsat a wavelength of 266 nm, the energy density per pulse applied to thedielectric material is between 0.2 J/cm² and 3 J/cm², inclusive, andpreferably is nominally 2 J/cm² for a pulse width of 100 ns or less.

Once the blind-via has been drilled through the dielectric layer to thefirst conductive layer, a blind-via post-pulse processing technique isused for enhancing the properties of the via. The output conditions ofthe laser are changed, preferably in situ over the drilled blind-via,and the first conductive layer is then laser drilled at the changedoutput conditions for a predetermined number of pulses, preferablybetween 1 to 10, inclusive. Each pulse applied to the first conductivelayer has an energy density per pulse that is greater than an ablationthreshold of the first conductive layer. The predetermined number ofpulses at the second energy density per pulse level is chosen so thatany dielectric material remaining on the surface of the first conductivelayer is ablated and so that the surface of the first conductive layerbecomes molten.

When the laser emits at a wavelength of 355 nm, the energy density perpulse applied to the first conductive layer is greater than 5 J/cm², andpreferably is nominally 11 J/cm² for a pulse width of 100 ns or less.When the laser emits at a wavelength of 266 nm, the energy density perpulse applied to the conductive layer is greater than 1.5 J/cm², andpreferably is nominally 5 J/cm² for a pulse width of 100 ns or less.

After the blind-via is filled with a conductive material, an averageresistance of an electrical connection through the blind-via to thefirst conductive layer is about 2 mΩ. A variance of the resistance ofthe electrical connection through the blind-via to the first conductivelayer is about 0.25 mΩ².

The present invention also provides a method for laser drillingthrough-vias in a laminated substrate where the through-vias have viaentrances of 75 μm or less, and aspect ratios of between 3:1 and 25:1.The laminated substrate is laser drilled from a top surface of thesubstrate to an exposed bottom surface of the substrate using aplurality of laser pulses that are preferably trepanned in a circle, butother trepanning patterns, such as ovals and squares, can be used. Eachpulse has an energy density per pulse that is sufficient for ablatingboth dielectric layers and conductive layers forming the laminatedsubstrate.

When the laser emits at a wavelength of 355 nm, the energy density perpulse applied to the substrate for drilling a through-via is greaterthan 2 J/cm², and preferably is nominally 10 J/cm² for a pulse width of100 ns or less. When the laser emits at a wavelength of 266 nm, theenergy density per pulse applied to the substrate for drilling athrough-via is greater than 2 J/cm², and preferably is nominally 10J/cm² for a pulse width of 100 ns or less.

The present invention also provides a method for enhancing the qualityof a via entrance, whether the via is a blind-via or a through-via, byapplying a polymeric photo-absorptive layer on an exposed top surface ofthe laminated substrate. The photo-absorptive layer is formed to bebetween 5 μm and 50 μm thick, inclusive, and is preferably about 25 μmthick. Ablated materials formed by laser drilling the substrate areredeposited on photo-absorptive layer surrounding the via. Thephoto-absorptive layer and the ablated material redeposited on thephoto-absorptive layer are then removed.

The exit variance of a through-via can also be enhanced by applying apolymeric photo-absorptive layer on the exposed bottom surface of thelaminated substrate that is between 5 μm and 50 μm thick, inclusive, andis preferably 25 μm thick. A layer of conductive material is then placedin intimate contact with the photo-absorptive layer formed on the bottomof the substrate. The through-via is drilled and the photo-absorptivelayer formed on the bottom surface of the substrate and the layer ofcopper are removed. By providing the photo-absorptive layer and copperlayer on the bottom surface of the laminated substrate, a variance ofthe exit width of the through-via of about 20 μm² is achieved when theaspect ratio of the through-via is about 10:1.

Once the through-via has been drilled through the substrate, the exitwidth variance can be further enhanced by performing one of severalthrough-via post-pulse processing techniques of the invention. In oneexample of a through-via post-pulse processing technique, the outputconditions of the laser are changed, preferably in situ over the drilledthrough-via, and the via is drilled a second time using a plurality oflaser pulses having the energy density per pulse initially used fordrilling the via. The laser pulses are then trepanned in a secondpredetermined pattern. Preferably, the second predetermined trepanningpattern is a circle having a diameter than is less that the diameter ofthe first circular trepanning pattern.

In a second example of a through-via post-pulse processing technique,the output conditions of the laser are changed, preferably in situ overthe drilled though-via, and the via is drilled a second time using aplurality of laser pulses having a greater energy density per pulse thanused for the initial drilling and that are trepanned in a secondpredetermined pattern at a trepanning rate that is less than that usedfor the initial drilling. Preferably, the trepanning rate, or spacingbetween pulses, for the initial drilling is between 0.5 μm and 6 μm,inclusive, while the second trepanning rate is 3 μm, or less, betweenpulses.

By performing either through-via post-pulse processing of the invention,high-aspect vias having exit widths of 75 μm, or less, are repeatablyproduced. For example, both through-via post-pulse processing techniquesprovide a variance of the exit width of the through-via of about 10 μm²when the aspect ratio of the through-via is 10:1. When the aspect ratioof the through-via is 20:1, a variance of the exit width of thethrough-via of about 15 μm² is achieved.

Since the entrance widths of the vias that are produced using thepresent invention are 75 μm, or less, and can have high aspect ratios,the present invention provides that an ultrasonic treatment inde-ionized water is applied to a drilled substrate for between 5 and 20minutes, inclusive, thereby removing material redeposited on sidewallsof the via.

The quality of the vias produced by the present invention are furtherensured by forming controlled thicknesses of copper oxide layers on theconductive copper layers so that dielectric layers better adhere to thecopper layers. The conductive layers are commercially available withpreformed oxide layers, or the oxide layers can be formed by immersingthe conductive layer in a brown oxide bath solution or a red oxide bathsolution, commercially available from McGean Rohco, at a temperature ofbetween 120° F. to 150° F. for between 30 seconds to 5 minutes, to forma copper oxide layer. When a via is drilled, the amount of energy usedis controlled so that thermal damage to the copper oxide layers anddielectric interface is minimal. After drilling, ablated substratematerial is removed by cleaning the substrate using minimal exposures toacidic cleaners so that the copper oxide layers and concomitantdielectric layer adhesion is not compromised.

The blind-via and through-via drilling and post-pulse processingaccording to the present invention can be performed a plurality of timesfor drilling a number of vias in a laminated substrate.

The present invention also efficiently forms a plurality of blind-viasin a laminated substrate by providing a method that laser drillsblind-vias in a laminated substrate preferably using a TEA CO₂ laser,and scanning the laser focal spot of the TEA laser for initiallydrilling the blind-vias. Afterward, each blind-via is laser drilledpreferably using a YAG laser and either a drill pulse or a trepanningmotion of a laser focal spot of the YAG laser that is within an entrancewidth of the blind-via. According to the invention, the YAG laser isselected from the group consisting of a frequency-tripled neodymium YAG(Nd:YAG) laser and a frequency-quadrupled Nd:YAG laser.

When a via is drilled with a laser having a wavelength of 266 nm, andthe via has an entrance width of 25 μm and a masking copper layer isemployed having a thickness of less than 10 μm, the present inventionprovides that a mask having an aperture is placed in the output beam ofthe laser at a distance from the output of the laser that is equal to orless than the Rayleigh range of the laser beam. The size of the apertureis selected so that side lobes of the laser beam are blocked fromreaching the exposed surface of the substrate.

A fiducial registration scheme is provided by the present invention sothat the individual conductive layers of a laminated substrate areformed without tolerance build-up problems. A registration mark isformed on the core layer of the substrate. The registration mark is thenused as a fiducial registration point for forming a first layer on thecore layer. The first layer is then laser drilled for exposing theregistration mark on the core layer. Afterward, a second layer is formedon the first layer using the registration mark as a fiducial point.Additional layers of the laminated substrate are formed by repeatingthese processes.

The present invention also provides fiducial registration verificationof conductive layers of a laminated substrate, preferably using kelvinresistors. Predetermined areas of resistive material are formed as partof a conductive layer and at a same corresponding location in eachrespective conductive layer. First and second voltage taps arerespectively formed at first and second ends of each predetermined area.The first and second voltage taps are separated by a first predetermineddesign distance that is preferably selected so that a predeterminedcurrent injected into the resistive material has a uniform currentdensity in the resistive material at the first and second voltage taps.A through-via is formed in the laminated substrate through therespective predetermined areas of resistive material and is connected toeach predetermined area of resistive material between the first andsecond ends of each respective predetermined area. A total resistance isdetermined between the first end and the second end of eachpredetermined area of resistive material. A fractional resistance isdetermined between the first end of each predetermined area of resistivematerial and the through-via. A fiducial misregistration of eachconductive layer is determined with respect to a location of thethrough-via based on the respective fractional resistance and the totalresistance for each respective predetermined area of resistive material.That is, the misregistration of each conductive layer is based on aratio of the fractional resistance of the conductive layer to the totalresistance of the conductive layer.

The predetermined current is passed from the first end to the second endof each predetermined area of resistive material and a voltage ismeasured between each respective first and second voltage tap. Eachrespective total resistance is determined based on the predeterminedcurrent and the voltage measured between each respective first andsecond voltage taps. A voltage is measured between each respective firstvoltage tap and the through-via, and the fractional resistance isdetermined based on the predetermined current and the voltage measuredbetween each respective first voltage tap and the through-via.

The misregistration δ for each conductive layer is determined withrespect to the location of the through-via by ##EQU1## where R₁ is thefractional resistance of the predetermined area of resistive materialbetween the first voltage tap and the through-via for a conductivelayer; R₂ is the fractional resistance of the predetermined area ofresistive material between the through-via and the second voltage tap;R_(T) is the total resistance of the predetermined area of resistivematerial of the conductive layer between the first and second voltagetaps, that is R₁ +R₂ ; and 1_(T) is the first predetermined designdistance separating the first and second voltage taps of the conductivelayer.

The present invention provides a method for producing a low inductancevia in a laminated substrate. A first dielectric layer is formed on afirst conductive layer, and a second conductive layer is formed on thefirst dielectric layer. A first conductive path is formed in the firstconductive layer that extends along a first route between a first nodeand a second node. A first blind-via is formed in the first dielectriclayer at the second node and is connected to the first conductive path.A second conductive path is formed in the second conductive layerconnected to the first blind-via. The second conductive path extendsbetween a third node and the first blind-via along a second route thatcorresponds identically to at least a portion of the first route. Amutual inductance formed by the first conductive path with the secondconductive path cancels a mutual inductance formed by the secondconductive path with the first conductive path.

The low inductance via can be expanded by forming a second dielectriclayer on the second conductive layer, a third conductive layer on thesecond dielectric layer, a third dielectric layer on the thirdconductive layer, and a fourth conductive layer on the third dielectriclayer. A first buried-via is formed in the second dielectric layer at athird node and is connected to the second conductive path. A thirdconductive path is formed in the third conductive layer that isconnected to the first buried via. The third conductive path extendsbetween a fourth node and the first buried via along a third route thatcorresponds identically to at least a portion of the second route. Asecond blind-via is formed in the third dielectric layer at the fourthnode and is connected to the third conductive path. A fourth conductivepath is formed in the fourth conductive layer and is connected to thesecond blind-via. The fourth conductive path extends between a fifthnode and the second blind-via along a fourth route that correspondsidentically to at least a portion of the third route. A mutualinductance formed by the second conductive path with the thirdconductive path cancels a mutual inductance formed by the thirdconductive path with the second conductive path. Similarly, a mutualinductance formed by the third conductive path with the fourthconductive path cancels a mutual inductance formed by the fourthconductive path with the third conductive path.

A plurality of low inductance vias can be formed by arranging theblind-vias and buried-vias of the low inductance conductive vias intorows and columns that are parallel to axes of a Cartesian coordinatesystem. The respective routes of the conductive paths are parallel witheach other and form an angle with the axes of the Cartesian coordinatesystem. Preferably, the respective routes form a 45° angle with the axesof the Cartesian coordinate system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the accompanying figures in which like reference numeralsindicate similar elements and in which:

FIG. 1 is a cross-sectional view of a laminated substrate having ablind-via and a through-via formed by a method of the present invention;

FIG. 2 is a photomicrograph showing a blind-via and a through-via formedby a method and an apparatus of the present invention;

FIG. 3 is a schematic view of a laser system for forming vias accordingto the present invention;

FIG. 4 is a graph showing laser output power as a function of pulserepetition rate;

FIGS. 5A and 5B are sectional views showing a multilayered substratehaving a blind-via formed therein, with FIGS. 5A and 5B showingdifferent stages of development of the blind-via;

FIGS. 6A and 6B are graphs showing average resistance of a blind-viawithout post-pulse treatment and with post-pulse treatment,respectively;

FIGS. 7A and 7B are sectional views showing a through-via in differentstages of development;

FIG. 8A is a sectional view of a multilayered substrate having a lowimpedance via pattern;

FIG. 8B is a schematic view of a current path which illustrates themutual impedance cancellation resulting from the pattern of vias;

FIG. 8C is a partial top plan view of a multilayered substrate showingthe pattern of vias that optimize density;

FIG. 9 is a top view of a kelvin resistor used as a fiducial checkaccording to the present invention;

FIG. 10 shows a cross-sectional view of a flip-chip MCM package using alaminated substrate according to the invention as an interconnectioncircuit device;

FIG. 11 is a scanning electron micrograph of a porous polymer used for afiller containing dielectric according to the present invention; and

FIG. 12 is a schematic of a porous polymer containing an adhesive filleraccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a laminated substrate 1 is constructed by serially,or sequentially, by laminating alternating layers of conductive anddielectric layers together. The various layers are positioned in a stackand then pressed together, usually with a dielectric material in ab-stage of curing so that the layers are not fully cured until afterpressing.

Laminated substrate 1 may include any number of layers, although FIG. 1illustrates a seven layer structure. Dielectric layers 3 and 4 aredisposed on opposite sides of the core layer 2, and conductive layers 5and 6 are disposed on the dielectric layers 3 and 4, respectively. Adielectric layer 7 is provided on the conductive layer 5, and adielectric layer 8 is provided on the conductive layer 6. A conductivelayer 9 is provided on the dielectric layer 7 and a conductive layer 10is provided on the dielectric layer 8.

The layers are applied serially such that at first the core layer 2,dielectric layers 3 and 4 and conductive layers 5 and 6 are pressed andbonded together. The conductive layers are patterned, and any necessaryblind-vias to connect conductive layers 3 and 4 are formed before theremaining layers are bonded to the structure. Subsequently, theadditional dielectric layers 7 and 8 and conductive layers 9 and 10 arebonded to the other layers as shown in FIG. 1.

Alternatively, several metal/dielectric/metal layers can besimultaneously pressed together, rather than being done in series. Thistype of assembly could obviate the need for a core layer, such as corelayer 2 in FIG. 1. Whether done serially or simultaneously, larger orsmaller numbers of layers can be employed. Seven and nine layersubstrates have many practical applications, as does the illustratedfive conductive layer substrate.

A blind-via 11 extends through dielectric layer 7 and a through-via 12extends through the entire laminated substrate 1. The laser drillingtechniques employed to form vias 11 and 12 will be described more fullybelow. Vias 11 and 12 have entrance diameters of between 10 μm and 75μm, inclusive. Further, through-vias formed using the methodology of thepresent invention are reliably and repeatably formed to have an aspectratio, that is, the ratio of via depth to via diameter D₁, of between3:1 to 25:1, inclusive.

FIG. 2 is a photomicrograph of a blind-via and a through-via that wereformed by the method of the present invention. The blind-via has anentrance diameter of 50 μm and an aspect ratio of 1:1, and thethrough-via has an entrance diameter of 50 μm and an aspect ratio of6:1. These are representative of the via sizes and aspect ratiosachievable using the methodology and apparatuses described herein.

Core layer 2 is made of a conductive material, such as a 1 oz. copperlayer having a nominal thickness of 38 μm. Other well-known conductivecore materials can also be used such as aluminum. Core layer 2 providesstructural support for the alternately disposed dielectric andconductive layers. Typically, core layer 2 is electrically grounded whenthe laminated substrate is assembled as an interconnection circuitdevice in an integrated circuit package.

The conductive and dielectric layers shown in FIG. 1 are disposedsymmetrically about core layer 2. That is, each dielectric or conductivelayer formed on one side of core layer 2 has a corresponding layer ofthe same material formed on the opposite side of core layer 2.Consequently, the following description sets forth a method for formingvias with respect to only one side of core layer 2, but a similarapproach is used for forming vias on the other side of core layer 2.Additionally, while the laminated substrate shown in FIG. 1 has only twodielectric layers and two conductive layers symmetrically formed on eachside of core layer 2, the laminated substrate of the present inventioncan have any number of alternately disposed dielectric and conductivelayers symmetrically formed on opposite sides of the core layer.

Dielectric layers 3, 4, 7, and 8 are preferably made from laminates ofhigh-temperature organic dielectric substrate materials, such as, butnot limited to, polyimides and polyimide laminates, epoxy resins,organic materials, or dielectric materials comprised at least in part ofpolytetrafluoroethylene, with or without a filler. A more detaileddescription of these materials is provided hereinbelow. Conductivelayers 5, 6, 9 and 10 are preferably formed from a conductive material,such as copper.

Dielectric layers 3 and 4 are laminated onto core layer 2 by placing thecore between two sheets of dielectric material and pressing themtogether. When core layer 2 is made of copper, copper oxide layers 2aand 2b, commonly known as a brown, black or red oxide, are preferablyprovided on opposite surfaces of core layer 2 for promoting adhesion ofdielectric layers 3 and 4 to core layer 2. Copper oxide layers 2a and 2bare formed on the core layer 2 by using standard surface treatmenttechniques, such as immersing the core layer in a brown oxide bath or ared oxide bath solution, commercially available from McGean Rohco, at atemperature of between 120° F. to 150° F. for between 30 seconds to 5minutes.

Dielectric layers 3 and 4 are formed from an organic substrate material,such as a high-temperature organic dielectric substrate material, tohave a thickness of between about 12 μm to 100 μm, inclusive. As arepresentative example, dielectric layers 3 and 4 could have a nominalthickness of about 50 μm.

Conductive layers 5 and 6 are stacked on top of dielectric layers 3 and4 and laminated together in the first pressing operation, in which core2, dielectric layers 3 and 4 and conductive layers 5 and 6 are pressedto form a laminated subassembly. In a second pressing operation,dielectric layers 7 and 8 and conductive layers 9 and 10 are stacked andpressed to form laminated substrate 1, shown in FIG. 1.

The conductive layers are made of a conductive material, preferably a1/2 oz. copper layer having a nominal thickness of 19 μm. In the finalinterconnection circuit device, conductive layers 5 and 6 are typicallypower layers, but can also be signal layers. Design and applicationconsiderations determine the package construction.

When conductive layers 5 and 6 are made of copper, copper oxide layers5a and 6a are formed on conductive layers 5 and 6, respectively, forpromoting adhesion of dielectric layers 7 and 8 to conductive layers 5and 6. Copper oxide layers 5a and 6a are formed on the conductive layers5 and 6, respectively, by using known techniques, such as immersing thesubstrate in a brown oxide bath or a red oxide bath solution,commercially available from McGean Rohco, at a temperature of between120° F. to 150° F. for between 30 seconds to 5 minutes.

Dielectric layers 7 and 8 are preferably formed from an organicsubstrate material, such as a high-temperature organic dielectricsubstrate material, to have a thickness of between about 12 μm to 100μm, inclusive. As a representative example, dielectric layers 7 and 8could have a nominal thickness of about 50 μm.

Conductive layers 9 and 10 are made of a conductive material, and arepreferably a 1/8 oz. copper layers having a nominal thickness of 5 μm.Typically, conductive layers 9 and 10 are conventional signal/pad layersin the final interconnection circuit device, but can also be power/padlayers.

Laser System for Forming Vias

The vias shown in FIG. 1 are formed using the laser system 20 shown inFIG. 3. Laser system 20 includes a laser source 22 which outputs apulsed laser beam 22a that is directed through a laser optic system. Theoptic system includes mirrors 24 and 26, and a focusing lens 28 whichdirects a focused beam through a patterned mask 30, used when blind-viasare formed, onto a workpiece 32 positioned on an X-Y positioning table34. The focused beam forms a focal spot on the substrate underlying themask 30. The focal spot can be circular or oval in shape.

Workpiece 32 of FIG. 3 is a laminated substrate that includes aplurality of alternately disposed dielectric and conductive layers, suchas the laminated substrate 1 shown in FIG. 1. After additionallaminations are applied, the blind-vias or through-vias formed in onestep become buried vias. Blind-vias can also be formed on thebottom-most or top-most dielectric layer, as exemplified by blind-via 11of FIG. 1.

Laser system 20 includes a controlling device, such as computer 36, thatcontrols the position of the focal spot of the laser beam with respectto workpiece 32. Computer 36 controls the positioning of the focal spotby issuing control signals Si to an actuator 38 which moves lens 28 inthe X direction. Computer 36 further issues control signals S2 to anactuator 40 which moves the positioning table 34 in the Y direction. Thecombined X and Y motion allows system 20 to move the laser beam in amanner programmed by computer 36 to form a desired type of via.

Other X-Y positioning may be employed, including galvanometers forcontrolling the position of mirrors 24 and 26. Galvanometers of thistype are commercially available and can be installed in commerciallyavailable laser apparatuses. In any event, computer 36 can bepre-programmed for establishing the pattern of motion of the beam spotto form the desired vias. Moreover, computer 36 is operatively coupledto laser source 22 for establishing lasing parameters such as direction,speed of the beam path (in millimeters per second), pulse repetitionrate, and power.

To adjust peak pulse power, computer 36 can implement a change in pulserepetition frequency, which affects peak power both by changes inaverage power and pulses per second, as well as the pulse duration inns. This is best done by changing pulse intervals with electronic oracousto-optic Q-switching. Power per pluse can also be changed byadjusting the current to the laser excitation source, such as in arclamp.

The preferred pattern of movement of the laser focal spot is"trepanning" the spot, in which the beam spot starts in the center ofthe desired via, and gradually spirals outwardly to an outer diameter ofthe via. At that point the beam is caused to orbit around the via centerfor as many revolutions as is determined necessary for the particularvia. Upon completion, the focal spot is caused to spiral back to thecenter and thereafter awaits the next command. An example of atrepanning velocity is 3 millimeters per second.

Choice of laser is important to the present invention. The preferredlasers are pulsed solid state lasers such as the frequency-tripledNd:YAG (neodymium yttrium aluminum garnet) laser emitting at a 355 nmwavelength or a frequency-quadrupled Nd:YAG laser emitting at a 266 nmwavelength. Such lasers are commercially available as the Model 5000Laser Processing System produced by Electro Scientific Industries ofPortland, Oreg., USA.

FIG. 4 is a graph showing an example of laser output power as a functionof pulse repetition frequency for laser source 22. As is evidenttherefrom, there is a non-linear relationship between average power andfrequency, but up to a maximum value of average power, the pulsefrequency can be increased to provide increased average power.

In changing the laser energy density in situ, the computer can controlthe Q-switch to vary the pulse repetition rate. Table 1 demonstrates howthe laser energy and power change with repetition rate for the 355 nmNd:YAG laser:

                  TABLE 1                                                         ______________________________________                                        Rep. Rate                                                                            Average    Pulse     Energy per                                                                            Power                                       (Hz) Power (mW) Width (nm) Pulse (mJ) per Pulse (kW)                        ______________________________________                                        2000   680        40        340     8.5                                         3000 770 48 255 5.3                                                           4000 770 55 195 3.5                                                           5000 725 63 145 2.3                                                           6000 685 70 115 1.6                                                           7000 630 78 90 1.2                                                            8000 580 85 75 .9                                                             9000 535 93 60 .6                                                             10000  500 100  50 .5                                                       ______________________________________                                    

Similarly, Table 2 demonstrates how the laser energy and power chargewith repetition rate for the 266 nm Nd:YAG laser:

                  TABLE 2                                                         ______________________________________                                        Rep. Rate                                                                            Average    Pulse     Energy per                                                                            Power                                       (Hz) Power (mW) Width (nm) Pulse (mJ) per Pulse (kW)                        ______________________________________                                        2000   330        40        165     4.1                                         3000 380 48 125 2.6                                                           4000 350 55 90 1.6                                                            5000 290 63 60 1.0                                                            6000 280 70 45 0.6                                                            7000 240 78 35 0.4                                                            8000 180 85 25 0.3                                                            9000 140 93 15 0.2                                                            10000  130 100  15 0.1                                                      ______________________________________                                    

When the laser focal spot geometry is added to the above, energy densitycan be calculated. The energy density is one of the primary parametersfor setting laser ablation of the substrates to form vias in the organicsubstrates described herein.

Method of Laser Drilling Blind-Vias

Referring to FIG. 5A, a laminated multilayered substrate 42 includes ablind-via 44 formed therein according to the methods and laser drillingapparatus of the present invention. Typically, blind-vias are formedthrough only one dielectric layer and are used for routing connectionsbetween two adjacent conductive layers. However, blind-vias can beformed using the present invention that go through a plurality oflaminated substrate layers in order to connect multi-conductive layers.

The range of aspect ratios for blind-vias that are achievable by thepresent invention is 1:1 to 5:1, inclusive. For example, a blind-via maybe formed having a via entrance width of 50 μm and extending through adielectric layer having a thickness of 50 μm (1:1).

As seen in FIG. 5A, laminated substrate 42 includes an outer conductivelayer 46 having a preformed aperture 46a formed at the location whereblind-via 44 is to be formed. Substrate 42 further includes a dielectriclayer 48, a conductive layer 50, a dielectric layer 52 and a conductivelayer 54. Aperture 46a is formed by conventional means, such as by achemical etching process, so that dielectric layer 48 is exposed throughaperture 46a. In particular, conductive layer 46 is coated with aphoto-absorptive material, and then the photo-absorptive materials areimaged with a pattern for aperture 46a. The imaged photo-absorptivelayer is developed and the exposed portion of conductive layer 46 ischemically etched to form aperture 46a.

After aperture 46a is formed, the photo-absorptive material used toimage conductive layer 46 and form opening 46a can be left on, ratherthan being etched away, to leave a polymeric photo-absorptive layer 56on conductive layer 46. Photo-absorptive layer 56 is between 5 μm and 50μm thick and, preferably, is nominally 25 μm thick. Suitablephoto-absorptive materials are any of a variety of commerciallyavailable photoresists.

With photo-absorptive layer 56 in place, substrate 42 is then placed onthe positioning table 34 of FIG. 3. The laser beam is positioned so thatlaser focal spot is focused to a predetermined spot size inside ofaperture 46a where blind-via 44 is to be drilled. The output powerlevel, the pulse repetition frequency, the pulse length or duration andlaser focal spot size are adjusted accordingly so that an energy densityper pulse is applied to substrate 42, and more specifically, todielectric layer 48, that is greater than an ablation energy densitythreshold of dielectric layer 48, but is less than an ablation energydensity threshold of conductive layer 50 (located below dielectric layer48).

When using a 355 nm laser source, the beam ablates material in theblind-via to a point where the beam begins to be partially reflected bythe underlying conductive layer 50, and thus, further exposure to thebeam forms an undesirable copper oxide due to local heating. At thispoint, a post-pulse processing technique is used for further processingblind-via 44 to completely remove any remaining dielectric material 48a.Complete removal provides a reduced resistance and resistance varianceof via 44.

For post-pulse processing, the output power level of laser 22 isincreased in situ over the drilled via to an energy density level perpulse that exceeds the ablation threshold of conductive layer 50 byadjusting the energy density of the laser beam. The number of pulses andpeak power of each pulse applied to conductive layer 50 at the increasedenergy density is selected so that the surface of conductive layer 50 atthe bottom of blind-via 44 becomes molten, but is not rapidly ablatedand drilled. Since the energy density per pulse has been increased, theremaining dielectric layer 48a is completely ablated.

The material from dielectric layer 48 that is ablated during laserdrilling is vaporized and pulled through a local source of exhaust, oris redeposited on polymeric photo-absorptive layer 56 surroundingaperture 46a. After via 44 achieves the depth shown in FIG. 5a,photo-absorptive layer 56 and the ablated material redeposited thereonare removed using known techniques, such as chemically stripping. Theblind-via shown in FIG. 2a was formed using a photo-absorptive layer onthe surface of the laminated substrate before laser drilling. When theenergy density per pulse applied to the substrate is larger than what isneeded for drilling a via, nodules, or beads, are formed on conductivelayer 48 around the via entrance that adversely affect the overall yieldand the reliability of an interconnection circuit device.

For the initial drilling of a blind-via, laser source 22 (FIG. 3) can beadjusted to have a desired power output and pulse repetition frequency,such as between 1 KHz and 15 KHz, inclusive.

When the laser is a tripled Nd:YAG laser emitting at a wavelength of 355nm, the energy density per pulse applied to dielectric layer 48 (FIG.5a) is between 0.5 J/cm² and 11 J/cm², and preferably is nominally 5J/cm². When dielectric layer 48 is made of a ePTFE matrix material, theenergy density per pulse is preferably between 3 J/cm² and 4 J/cm². Whendielectric layer 48 includes a filler, the energy density per pulse ispreferably about 7 J/cm². When the laser emits at a 355 nm wavelength,the laser is preferably adjusted to have a pulse repetition frequency of6000 Hz, a pulse width of 70 ns, and a spot size of 35 μm for a 50 μmdiameter via entrance. Pulse widths, as used herein, neglect rise andfall times of a pulse. Spot sizes, as used herein, are approximately1/e² sizes. Other pulse repetition frequencies may also be used withpulse widths that are typically less than 100 ns, as long as asufficient energy density per pulse for ablating dielectric layer 48 isapplied.

When the laser is a quadrupled Nd:YAG laser emitting at a wavelength of266 nm, the energy density per pulse applied to dielectric layer 48 isbetween 0.2 J/cm² and 3 J/cm², and preferably is nominally 2 J/cm². Whenthe laser emits at a 266 nm wavelength, it is preferably adjusted tohave a pulse repetition frequency of 7000 Hz, a pulse width of 78 ns,and a spot size of 30 μm for a 50 μm diameter via mask entrance. Otherpulse repetition frequencies may also be used with pulse widths that aretypically less than 100 ns as long as a sufficient energy density perpulse for ablating dielectric layer 48 is applied.

The laser beam is preferably applied to dielectric layer 48 withinaperture 46a without trepanning, that is, moving the laser focal spot asthe blind-via is being drilled. Alternatively, the laser focal spot canbe trepanned in a circular motion within aperture 46a to form a viahaving a circular cross-section. Specifically, the focal spot isinitially focused to the center point of where the via is to be drilled.As the laser is pulsed, lens 28 and table 34 move in a coordinatedfashion so that the focal spot is spirally trepanned outward to a circlehaving a predetermined diameter, and then trepanned around the circlefor as many times as are needed to drill the via, and then spirallytrepanned back to the center point before the operating conditions ofthe laser are changed, or the focal spot is moved to another via. Therate of trepan and the corresponding spacing between each pulse isrelated to the pulse repetition frequency and the per-pulse power leveloutput from the laser.

For post-pulse processing, the laser power density, repetition rates,and pulse lengths are varied according to specific needs and drillingoperations. Some representative values for different lasers forblind-via post-pulse processing are provided in detail hereinafter.

When the laser 22 is a tripled Nd:YAG laser emitting at a wavelength of355 nm, the energy density per pulse applied to conductive layer 50 forpost-pulse processing is greater than 5.5 J/cm², and preferably isnominally 11 J/cm². At this wavelength, the laser is adjusted to have apulse repetition frequency of 4000 Hz, and a pulse length of 55 ns. Afocal spot size of 35 μm is employed. Typically, between 1 and 10pulses, inclusively, are used for 355 nm post-pulse processing. Otherpulse repetition frequencies may also be used with pulse widths that aretypically less than 100 ns as long as a sufficient energy density perpulse for ablating conductive layer 50 is applied to conductive layer.

When the laser 22 is a quadrupled Nd:YAG laser emitting at a wavelengthof 266 nm, the energy density per pulse applied to conductive layer 50is greater than 1.5 J/cm², and preferably is nominally 5 J/cm². When thelaser 22 emits at a 266 nm wavelength, the laser beam is preferablyadjusted to have a pulse repetition frequency of 5000 Hz, a pulse lengthof 63 ns, and a spot size of 30 μm. Similarly, between 1 and 10 pulses,inclusively, are used for 266 nm post-pulse processing. Other pulserepetition frequencies may also be used with pulse lengths that aretypically less than 100 ns as long as a sufficient energy density perpulse for ablating dielectric layer 50 is applied.

After post-pulse processing, and as shown in FIG. 5B, blind-via 44 isplated with a conductive material which forms a conductive layer 58using known techniques such as electroless plating. This platingprovides an electrical inter-connection between the two adjacentconductive layers 46 and 50.

Blind-vias post-pulse processed according to the methodology of thepresent invention have a reduced average resistance after the conductivematerial is plated into the blind-via as compared to blind-vias thathave not been post-pulse processed. When vias not receiving thepost-pulse processing according to the present invention were tested, anaverage resistance of a blind-via was measured at about 3 mΩ. Withpost-pulse processing, a comparably produced blind-via had an averageresistance of about 2 mΩ. Data taken from the tests are roughlyreproduced in graphic form in FIGS. 6A and 6B, which show averageresistance for different power levels per pulse for blind-vias with(FIG. 6B) and without (FIG. 6A) post-pulse processing. In FIG. 6A, theaverage resistance of a blind-via produced with no post-pulse processingis 3.2 mΩ with a 3a point of 7.7 mΩ. Additionally, the data showed thatthe variance of blind-via resistance, an important process factor, usingpost-pulse processing according to the present invention, is reducedfrom 2.4 mΩ² to 0.3 mΩ².

FIG. 6B graphically shows that with post-pulse processing the averageresistance is 2.4 mΩ with a 3σ point of 3.9 mΩ². Thus, post-pulseprocessing of the present invention reduces the average resistance of ablind-via and significantly reduces the distribution of resistancesobtained, providing extremely high part yields and well-controlledresistive qualities of blind-vias in an interconnection circuit device.

Tables 3-12 set forth results of a test in which multiple panels oflaminated substrates were laser drilled for producing blind-vias havingvia entrance diameters of 50 μm and an aspect ratio of 1:1 in aSpeedboard® C dielectric material, using a quadrupled Nd:YAG laseremitting at a 266 nm wavelength and with a 25 μm spot size. Theexperimental matrix was a full factorial design comprising 108 blind-vialaser conditions. Each group of vias per laser drilled conditions usedone of three laser output energy levels, 175 mW, 200 mW and 225 mW.Power was set for the laser rail at 3000 Hz. Each group of blind-viaswas subdivided into 3 subgroups that were drilled at one of three pulserepetition frequencies, 6000 Hz, 7000 Hz and 8000 Hz. Pulse width variedbetween 70 ns for 6000 Hz to 85 ns for 8000 Hz, so that between 5 μJ perpulse and 15 μJ per pulse was applied to the dielectric. Each subgroupof vias was further subdivided into 3 sub-sub groups that were drilledusing one of three predetermined number of pulses, 75, 150 and 225. Onehalf of each sub-subgroup was post-pulse processed, while the other halfwas not post-pulse processed. Post-pulse processing changed the laserconditions in situ over a blind-via so that about 25 μJ per pulse wasapplied to the substrate.

The average resistance for both panels for all conditions was measuredto be 2.83 mΩ. The average resistance for both panels for non-post-pulseprocessed blind-vias was measured to be 3.22 mΩ. The average resistancefor both panels for post-pulse processed blind-vias was measured to be2.43 mΩ. Table 3 sets forth the average resistance measured for each ofthe three different energy levels. Table 3 generally shows that anoutput energy level of 175 mW did not sufficiently ablate all dielectricmaterial from a blind-via, resulting in a higher measured resistance.

                  TABLE 3                                                         ______________________________________                                        POWER                                                                           SETTING LEVEL AVERAGE                                                         @ 3000 Hz RESISTANCE                                                          (mW) (mΩ)                                                             ______________________________________                                        175             4.11                                                            200 2.26                                                                      225 2.11                                                                    ______________________________________                                    

Table 4 sets forth the average resistance measured for each of the threedifferent pulse repetition frequencies. Table 4 shows that a higherpulse repetition frequency, with a correspondingly longer pulse widthand lower power level, did not sufficiently ablate the dielectricmaterial from a blind-via, resulting in a higher measured resistance.

                  TABLE 4                                                         ______________________________________                                        PULSE                                                                           REPETITION AVERAGE                                                            FREQUENCY RESISTANCE                                                          (Hz) (mΩ)                                                             ______________________________________                                        6000           1.73                                                             7000 2.23                                                                     8000 4.51                                                                   ______________________________________                                    

Table 5 sets forth the average resistance measured for each of the threedifferent predetermined number of pulses used for drilling a via. Table5 shows that when 75 pulses were used for drilling a via for all outputenergy levels, the dielectric material was not sufficiently ablated froma blind-via, resulting in a higher measured resistance.

                  TABLE 5                                                         ______________________________________                                                       AVERAGE                                                          NUMBER OF RESISTANCE                                                          PULSES (mΩ)                                                           ______________________________________                                         75            3.89                                                             150 2.36                                                                      225 2.22                                                                    ______________________________________                                    

Table 6 sets forth the average resistance measured for no post-pulseprocessing and for post-pulse processing. Table 6 shows that post-pulseprocessing decreased the average resistance for all output energy levelsand all pulse repetition frequencies used.

                  TABLE 6                                                         ______________________________________                                                       AVERAGE                                                          POST-PULSE RESISTANCE                                                         PROCESSING (mΩ)                                                       ______________________________________                                        NO             3.22                                                             YES 2.43                                                                    ______________________________________                                    

Table 7 sets forth the average resistance measured as a function ofoutput energy level and pulse repetition frequency. Table 7 shows that alower pulse repetition frequency, with a correspondingly shorter pulsewidth and higher power level, provided a lower average resistance.

                  TABLE 7                                                         ______________________________________                                        POWER          PULSE                                                            SETTING LEVEL REPETITION AVERAGE                                              @ 3000 Hz FREQUENCY RESISTANCE                                                (mW) (Hz) (mΩ)                                                        ______________________________________                                        175            6000       1.82                                                  175 7000 2.51                                                                 175 8000 7.99                                                                 200 6000 1.66                                                                 200 7000 2.11                                                                 200 8000 3.00                                                                 225 6000 1.71                                                                 225 7000 2.08                                                                 225 8000 2.54                                                               ______________________________________                                    

Table 8 sets forth the average resistance measured as a function ofoutput energy level and number of pulses used. Table 8 shows that alower output energy level and a lower number of pulses used for drillingprovided a higher average resistance.

                  TABLE 8                                                         ______________________________________                                        POWER SETTING             AVERAGE                                               LEVEL @ 3000 Hz NUMBER OF RESISTANCE                                          (mW) PULSES (mΩ)                                                      ______________________________________                                        175            75         6.93                                                  175 150 2.83                                                                  175 225 2.56                                                                  200 75 2.53                                                                   200 150 2.17                                                                  200 225 2.07                                                                  225 75 2.23                                                                   225 150 2.09                                                                  225 225 2.03                                                                ______________________________________                                    

Table 9 sets forth the average resistance measured as a function ofwhether post-pulse processing was used. Table 9 shows that use ofpost-pulse processing decreased average resistance for all output energylevels used. As previously mentioned. FIGS. 6A and 6B show a graph ofaverage resistance for different power levels per pulse for blind-viaswith and without post-pulse processing for the data of Table 9.

                  TABLE 9                                                         ______________________________________                                        POWER SETTING             AVERAGE                                               LEVEL @ 3000 Hz POST-PULSE RESISTANCE                                         (mW) PROCESSING (mΩ)                                                  ______________________________________                                        175            NO         4.89                                                  175 YES 3.31                                                                  200 NO 2.49                                                                   200 YES 2.02                                                                  225 NO 2.27                                                                   225 YES 1.96                                                                ______________________________________                                    

Table 10 sets forth the average resistance measured as a function ofpulse repetition frequency and whether post-pulse processing was used.Table 10 shows that post-pulse processing decreased the averageresistance for all pulse repetition frequencies used.

                  TABLE 10                                                        ______________________________________                                        PULSE                     AVERAGE                                               REPETITION POST-PULSE RESISTANCE                                              FREQUENCY (Hz) PROCESSING (mΩ)                                        ______________________________________                                        6000           NO         1.83                                                  6000 YES 1.64                                                                 7000 NO 2.41                                                                  7000 YES 2.06                                                                 8000 NO 5.42                                                                  8000 YES 3.60                                                               ______________________________________                                    

Table 11 sets forth average resistance measured as a function of numberof pulses used and whether post-pulse processing was used. Table 11shows that post-pulse processing decreased the average resistance forthe different total of pulses used.

                  TABLE 11                                                        ______________________________________                                        NUMBER OF     POST-PULSE AVERAGE                                                PULSES PROCESSING RESISTANCE (mΩ)                                     ______________________________________                                         75           NO         4.58                                                    75 YES 3.21                                                                  150 NO 2.61                                                                   150 YES 2.11                                                                  225 NO 2.46                                                                   225 YES 1.98                                                                ______________________________________                                    

Table 12 sets forth the standard deviation for the average resistancemeasured for whether post-pulse processing was used. Table 12 shows thatpost-pulse processing reduced the standard deviation of resistancesmeasured by a factor of 3. FIG. 6A graphically shows that the averageresistance of a blind-via produced with no post-pulse processing is 3.2mΩ with a 3σ point of 7.7 mΩ. FIG. 6B graphically shows that withpost-pulse processing the average resistance is 2.4 mΩ with a 3σ pointof 3.9 mΩ.

                  TABLE 12                                                        ______________________________________                                        POST-PULSE   AVERAGE RESISTANCE                                                 PROCESSING (mΩ)                                                       ______________________________________                                        NO           1.54                                                               YES 0.54                                                                    ______________________________________                                    

While the laser power density, repetition rates, and pulse widths can bevaried according to specific needs and drilling operations, somerepresentative values for the 355 nm Nd:YAG laser with a 35 μm spot sizeare provided as follows in Table 13.

                  TABLE 13                                                        ______________________________________                                        Dielectric  Energy per                                                                              Energy Density                                                                           Power Density                                  Material Pulse (μJ) (J/cm.sup.2) (MW/cm.sup.2)                           ______________________________________                                        CE-epoxy/PTFE                                                                             30        3.5        35                                             post-pulse 100 11 200                                                         CE- 65 7.0 100                                                                epoxy/PTFE/ceramic                                                            filler                                                                        post-pulse 100 11 200                                                       ______________________________________                                    

Similarly, for the 266 nm wavelength Nd:YAG laser, representative valuesfor forming a blind-via with a 30 μm focal spot are as follows in Table14.

                  TABLE 14                                                        ______________________________________                                        Dielectric                                                                              Energy per Energy Density                                                                            Power Density                                  Material Pulse (μJ) (J/cm.sup.2) (MW/cm.sup.2)                           ______________________________________                                        CE-epoxy/PTFE                                                                           10         1.5         20                                             post-pulse 20 3 40                                                          ______________________________________                                    

For dielectric materials having ceramic filler, higher values arerequired. For instance, the energy density applied to conductive layer50 is greater than 4 J/cm², and preferably is nominally 9 J/cm². Thesevalues correspond to a pulse repetition frequency of 5000 Hz, a pulselength of 55 ns, and a spot size of 30 μm. For the post-pulse step,between 1 and 10 pulses, inclusively, are used. Other pulse repetitionfrequencies may also be used with pulse lengths that are typically lessthan 100 ns as long as a sufficient energy density per pulse forablating dielectric layer 50 is applied.

Forming blind-vias according to the methodology described aboveminimizes damage to the mask (layer 30 in FIG. 3), as well as the copperoxide layers associated with conductive layers 2a, 2b, 5a and 6a of FIG.1 and those that would be associated with conductive layers 50 and 54 ofFIG. 5A and 5B, the latter being excluded from illustration for clarity.The minimized damage results from minimizing the amount of peak powerused for drilling a via. The copper oxide layer is not partially etchedback when the laminated substrate is cleaned after laser drilling in anacidic cleaner, thus resulting in a more reliable via because the copperand dielectric layers in the substrate remain adhered to each other inthe vicinity of the via.

Methods For Forming Through-Vias

The present invention further entails the formation of through-viasusing the laser drilling apparatus shown in FIG. 3. It is possible,using the present techniques, to drill through-vias having entrancediameters of 75 μm or less and aspect ratios of between 3:1 and 25:1.

Referring to FIG. 7A, a laminated substrate 60 is shown with athrough-via 62 passing through alternating layers of dielectric andconductive layers. When the through-via 62 is drilled, the conductiveand the dielectric layers are drilled at the same time, as describedbelow.

Laminated substrate 60 in FIG. 7A is formed with a polymericphoto-absorptive coating 64 on an upper most conductive layer 66. Thisphoto-absorptive coating is the same type described with respect to FIG.5A, and can be applied using a roll lamination, a spray coating, or aspin-coating technique, for example. Coating 64 is between 5 μm and 50μm thick and, preferably, nominally 25 μm thick. Suitablephoto-absorptive materials for the coating are commercially available.When the photo-absorptive layer is removed after laser drilling, the viaentrance is of high-quality since ablated material deposits on layer 64and is removed.

To form a via exit of high quality, that is, a low exit width variance,a polymeric photo-absorptive layer 68 is formed on the bottom mostexposed conductive layer 70 using a known technique, such as a rolllamination, a spray coating or a spin coating process. Photo-absorptivelayer 68 has a thickness of between 5 μm to 50 μm, and preferably has anominal thickness of 25 μm. A series of intermediate alternating layersof conductive layers 72, 74, and 76 and dielectric layers 78, 80, 82 and84 are provided between the upper most and lower most conductive layers66 and 70.

It is important to maintain planarity of the bottom side of laminatedsubstrate 60 during the process of laser drilling to form through-via62. Conventional approaches of holding a laminated substrate against ascreen or a honeycomb grid using a vacuum does not provide the requiredcombination of sufficient bottom side planarity and minimal redeposit ofmaterial conformal to the bottom side of the substrate. Most vacuumplates are fabricated with aluminum or steel, thus providing a potentialfor redeposit of metal that is difficult to remove. In some instances,photo-absorptive layer 68 alone is sufficient for providing high-qualityvia exits. To ensure both an easy to remove redeposit material with viasidewalls, as well as the necessary bottom side planarity, a layer 86made of conductive material, such as copper, is placed in intimatecontact with photo-absorptive layer 68 by taping substrate 60 coatedwith photo-absorptive layer 68 to a flat conductive plate, such ascopper.

To laser drill through-via 62 shown in FIG. 7A, substrate 60 ispositioned so that the laser focal spot is focused to predetermined Xand Y coordinates where through-via 62 is to be drilled. For athrough-via having a 50 μm diameter, the output power level, the pulserepetition frequency, the pulse width and laser focal spot size of thelaser are adjusted accordingly so that an energy density per pulse isapplied to substrate 60 that is greater than an ablation energy densitythreshold of the conductive layers 66, 72, 76 and 70. For example, thecomputer of the laser system shown in FIG. 3 sets the power output andthe pulse repetition frequency between 1 KHz and 15 KHz, inclusive, anda pulse length of between 40 ns and 100 ns, inclusive, utilizing a laserspot size of between 25 μm and 35 μm in diameter, for example, to ablatethe layers of substrate 60.

When the laser is a tripled Nd:YAG laser emitting at a wavelength of 355nm, the energy density per pulse applied to substrate 60 is greater than2 J/cm², and preferably is nominally 10 J/cm². For a 355 nm wavelengthoutput, the laser is preferably adjusted to have a pulse repetitionfrequency of 8000 Hz, a pulse width of 85 ns and a spot size of 35 μm.Other pulse repetition frequencies may also be used with pulse widthsthat are typically less than 100 ns as long as a sufficient energydensity per pulse for ablating the layers of the substrate 60 isapplied.

Some other representative through-via forming parameters for the 355 nmwavelength laser are as follows in Table 15:

                  TABLE 15                                                        ______________________________________                                        Dielectric                                                                              Energy per Energy Density                                                                            Power Density                                  Material Pulse (μJ) (J/cm.sup.2) (MW/cm.sup.2)                           ______________________________________                                        ceramic filled                                                                          75         8           95                                             CE-epoxy/PTFE                                                                 CE-woven 200 22 400                                                           glass/CE-                                                                     Epoxy/PTFE                                                                  ______________________________________                                    

When the laser is a quadrupled Nd:YAG laser emitting at a wavelength of266 nm, the energy density per pulse applied to the substrate is greaterthan 2 J/cm², and preferably is nominally 10 J/cm². For a 266 nmwavelength output, the laser is preferably adjusted to have a pulserepetition frequency of 5000 Hz, a pulse width of 55 ns and a spot sizeof 25 μm. Other pulse repetition frequencies may also be used with pulsewidths that are typically less than 100 ns as long as a sufficientenergy density per pulse for ablating the layers of the substrate 60 isapplied. Other representative values for through-via formation using a266 nm wavelength Nd:YAG laser with a 25 mm focal spot are as follows inTable 16.

                  TABLE 16                                                        ______________________________________                                        Dielectric                                                                              Energy per Energy Density                                                                            Power Density                                  Material Pulse (μJ) (J/cm.sup.2) (MW/cm.sup.2)                           ______________________________________                                        ceramic filled                                                                          50         10          150                                            CE-epoxy/PTFE                                                               ______________________________________                                    

To form the through-via of FIG. 7A, the laser beam is preferably appliedto substrate 60 in a trepanned motion as was described with respect toformation of the blind-via. This results in the formation of athrough-via having a circular cross-section. The trepanning motioncontinues for as long as is necessary to drill the through-via, and thenthe focal spot is spirally trepanned back to the center point before theoperating conditions of the laser are changed, or focal spot is moved toanother via. To form a 50 μm diameter via, the center of a 25 μmdiameter focal spot is trepanned in a 40 μm diameter circular pattern ata trepanning distance of 0.8 to 6 μm between pulses.

When through-via 62 is drilled through to the bottom side of substrate60, minimal redeposit on the sidewalls occurs due to the high UV-VIS(ultraviolet-visible) absorptivity of the polymer coating on the viaexit and conductive layer 86 after ablation-redeposit. Once drilling iscomplete, conductive layer 86 is separated from substrate 60 andphoto-absorptive layer 68 is stripped away using known techniques. Also,photo-absorptive coating 66 is stripped, resulting in substrate 60 shownin FIG. 7B.

Use of photo-absorptive layer 68 formed on the bottom surface ofsubstrate 60 and conductive layer 86 held in intimate contact with thesubstrate ensures the formation of through-vias having a low exit widthvariance. For example, for through-vias having an aspect ratio of about10:1, an exit width variance of about 20 μm² has been measured. Asanother example, an exit width variance of about of about 30 μm² hasbeen measured for through-vias having an aspect ratio of about 20:1.

Multi-frequency Processing and Multiple Pulse Spacing

Many times when a through-via is initially drilled, the via exit openingat the bottom conductive layer 70 (FIG. 7A and 7B) is smaller than thevia entrance opening at the upper conductive layer 66. Both nominal exitdiameter and through-via exit width variance can be further improved byperforming post-pulse processing. That is, the laser system outputconditions are changed in situ over through-via 62 after the via isdrilled in one of four ways and the laser beam is then trepanned for anadditional trepanning pass typically using a smaller trepanning circlepattern.

To illustrate the four different through-via post-processing techniques,the situation where a 50 μm diameter through-via is drilled isconsidered. The through-via is initially drilled using an energy densityper pulse of 50 μJ/pulse applied to the substrate at a pulse repetitionrate of 5000 Hz, using a 25 μm spot size and a 40 μm diameter circulartrepanning pattern at a trepanning distance of between pulses of 0.8 to6 μm per pulse. In both through-via post-processing techniques, thelaser output conditions are changed in situ over the through-via foravoiding tolerance build-up problems associated with repositioning thelaser beam spot across the substrate and within entrances of the drilledthrough-vias. Re-lasing with multiple registration operations may resultin severe entrance nodules and overall poor via quality.

For the first through-via post-pulse processing technique, the laseroutput conditions are adjusted in situ over the just-completedthrough-via by keeping the energy density per pulse constant at 50μJ/pulse, the pulse repetition rate constant at 5000 Hz, the spot sizeconstant at a 25 μm diameter, the trepanning rate constant at 0.8 to 6μm spacing per pulse, but the trepanning pattern is reduced to be a 30μm diameter circle.

For the second through-via post-pulse processing technique, the laseroutput conditions are adjusted in situ over the just-completedthrough-via by keeping the energy density per pulse constant at 50μJ/pulse, the pulse repetition rate constant at 5000 Hz, the spot sizeconstant at a 25 μm diameter, and by reducing the trepanning rateconstant by a factor of 2.0 or greater to 0.4 to 3 μm spacing per pulse,and reducing the trepanning pattern to be a 30 μm diameter circle.

For the third through-via post-pulse processing technique, the laseroutput conditions are adjusted in situ over the just-completedthrough-via by increasing the energy density per pulse to 60 to 75μJ/pulse, keeping the trepanning rate to be between 0.8 μm and 6 μm perpulse, and reducing the trepanning pattern to be a 30 μm diametercircle. This can be accomplished by changing the pulse repetition rateto approximately 4500 Hz.

For the fourth through-via post-pulse processing technique, the laseroutput conditions are adjusted in situ over the just-completedthrough-via by increasing the energy density per pulse to 60 to 75μJ/pulse, reducing the trepanning rate by a factor of two or greater to0.4 μm and 3 μm per pulse, and reducing the trepanning pattern to be a30 μm diameter circle.

Through-via post-pulse processing provides a significant improvement inopen through-via yield, while also providing an even further improvedthrough-via exit width variance when compared to through-vias that arenot post-pulse processed. For example, when the aspect ratio of thethrough-via is 5:1, a variance of about 5 μm² is achieved. Similarly,when the aspect ratio of the through-via is 10:1, a variance of the exitwidth of the through-via of about 10 μm² is achieved using through-viapost-pulse processing. Similarly, when the aspect ratio of thethrough-via is 20:1, a variance of the exit width of the through-via ofabout 15 μm² is achieved.

The taper of the sidewalls of a via can be varied and is a function ofthe power level used to drill a via and the materials used in thesubstrate, the pulse step distance, and the total energy per via. Taper,for purposes of this disclosure is defined as: ##EQU2## where, D₁ is theentrance diameter of the a via, and D₂ is the exit diameter of the via.These diameters are shown in FIG. 1. Taper, defined in this manner,varies between 0 and 1. A taper of 0 indicates that the exit diameterequals the entrance diameter. A taper of 1 indicates that the via didnot reach its destination. Taper for through-vias having an aspect ratioof 10:1 that are drilled in substrates having dielectric layers formedfrom paste composite dielectric materials can be formed having a taperfrom 0.0 to 0.4.

The laser conditions used for achieving this taper are initiallydrilling at a wavelength of 355 nm, an energy density per pulse of 75μJ/pulse, a pulse repetition rate of 8000 Hz, a spot size of 35 μm, anda 40 μm diameter circular trepanning pattern at a trepanning rate of 2μm to 3 μm between pulses. The resulting taper is approximately 0.3.Through-via post-pulse processing by either alternative discussed abovecan be used after the initial drilling.

Through-vias having an aspect ratio of 10:1 drilled in substrates havingdielectric layers formed from woven glass dielectric materials can beformed having a taper of 0.2 to 0.5. The laser conditions used forachieving this taper are initially drilling at a wavelength of 355 nm,an energy density per pulse of 200 μJ/pulse, a pulse repetition rate of3500 Hz, a spot size of 35 μm, and a 40 μm diameter circular trepanningpattern at a trepanning rate of 4 to 6 μm per pulse. Through-viapost-pulse processing by either alternative is used after the initialdrilling.

When a laser emitting at a wavelength of 266 nm is used, the taperachieved for a 10:1 ratio via for substrates having a ceramic/CE-epoxyPTFE dielectric material is 0.0 to 0.2. The laser conditions used forachieving this taper are initially drilling at a wavelength of 266 nm,an energy density per pulse of 50 μJ/pulse, a pulse repetition rate of5000 Hz, a spot size of 25 μm, and a 40 μm diameter circular trepanningpattern at a trepanning rate of 2 μm to 3 μm between pulses. Theresulting taper is approximately 0.1. Through-via post-pulse processingby either alternative discussed above can be used after the initialdrilling.

Through-vias having an aspect ratio of 10:1 drilled in substrates havingdielectric layers formed from woven glass dielectric materials can beformed having a taper of 0.1 to 0.3. The laser conditions used forachieving this taper are initially drilling at a wavelength of 266 nm,an energy density per pulse of 50-75 μJ/pulse, a pulse repetition rateof 4000-6000 Hz, a spot size of 35 μm, and a 40 μm diameter circulartrepanning pattern at a trepanning rate of 1 to 6 μm per pulse.Through-via post-pulse processing by either alternative is used afterthe initial drilling.

Cleaning Blind-vias and Through-vias

After both blind-vias and through-vias have been laser drilled by themethods described above, and the photo-absorptive layers have beenremoved, a cleaning step is initiated. Since the via entrances formed bythe present invention are 75 μm and less, conventional via cleaning doesnot remove the ablated material redeposited on the sidewalls of the viasto the degree necessary for reliably plating conductive material intothe vias.

According to the present invention, an aggressive ultrasonic treatmentin de-ionized water is used to lean the vias. The ultrasonic treatmentis typically carried out for between 5 and 20 minutes and is applied tothe substrate for removing ablated material redeposited on sidewalls ofthe vias.

TEA & YAG Laser Processing

When a large number of blind-vias need to be drilled in a particularlayer of a laminated substrate, that is, equal to or greater than 1500blind-vias in an area about 45 mm², or a greater than or equivalent viadensity of 0.75 vias/mm², the blind -via laser processing describedabove can be implemented using a transversely excited atmospheric(pressure) (TEA) CO₂ laser in a scan mode operation for initiallydrilling each blind-via, and then using a solid state 3rd or 4th YAGlaser in a drill mode operation for performing post-pulse processing forenhancing the qualities of each blind-via and reducing the averageresistance and resistance variance of each blind-via. For example, thelaser 22 of FIG. 3 can be a 60 W TEA CO₂ laser having a wavelength ofabout 9 μm to 11 μm, a pulse repetition frequency of about 150 Hz, and apulse width of about 100 ns can be used in a scan mode operation forinitially drilling 50 μm diameter blind-vias in a 50 μm thick dielectriclayer. In this example, the TEA CO₂ laser is adjusted to apply 26 J/cm²per pulse so that the initial drilling is a 2 or 3 pulse operation perblind-via. After all blind-vias have been drilled by the TEA laser, thesubstrate is switched over to a suitable pulsed YAG laser for blind-viapost-pulse processing for reducing the average resistance and thevariance of the resistance of the vias, as described above.

Correcting Astigmatism

When a quadrupled Nd:YAG laser (wavelength equals 266 nm) is used forlaser drilling, as described above, the entrance quality of the vias canbe ensured by correcting for astigmatism in the laser beam. Referringagain to FIG. 3, a plate 41 is interposed in the beam path between lasersource 22 and substrate or workpiece 32. In particular, beam 22a passesthrough an aperture formed in plate 41. Plate 41 is positioned in thebeam path at a point that is equal to or less than the Rayleigh range ofthe beam. The size of aperture 41 is selected to block the side lobes ofbeam 22a.

Fiducial Laser Drilling

When producing a laminated substrate, the laser drilling techniques ofthe present invention can be used for determining the fiducialregistration of each conductive layer. At least two registration marks,such as circle shapes, are formed on core layer 2 shown in FIG. 1. Aseach layer is formed, laser drilling and post-pulse processing is usedfor drilling down to core layer 2 for exposing the registration marks sothat the registration marks formed on core 2 can be used as fiducialregistration points for aligning the next layer. The process of drillingeach layer to expose the fiducial registration marks is done for alllayers except for the final outer conductor layer, which is formed usingknown self-aligning techniques.

Forming Low Inductance Vias

The present invention includes a method for making a low inductanceconductive via. FIG. 8A shows a cross-sectional view of a multilayeredsubstrate 88 having low inductance vias, based on ablind-buried-blind-via pattern. The substrate is formed by layeringdielectric layers 90, 92, and 94 and conductive layers 96, 98, 100, and102. These layers are made of the materials and by the processesdescribed above. A buried via 104 provides an electrical interconnectionbetween adjacent conductive layers 98 and 100, and was formed by themethods described above with respect to forming blind-vias, prior tobeing enclosed by subsequent layering.

Blind-vias 106 and 108 are formed opposite of each other and fromopposite sides of the substrate 88. These are formed according to themethods described above, and correspond to blind-via 11 of FIG. 1 andblind-via 44 of FIGS. 5A and 5B.

A signal path is schematically illustrated in FIG. 8B, showing flow fromconductive layer 96, through-via 106 to layer 98, through layer 98,through buried-via 104 to layer 100, through layer 100, throughblind-via 108 to layer 102, and through layer 102.

This signal path is arranged such that portions of the conductive layersare routed along parallel but vertically spaced paths, with the currentor signal flowing in opposite directions. By arranging the signals inthis manner, a mutual inductance formed by a first signal path segmentwith a second adjacent signal path segment cancels a mutual inductanceformed by the second conductive path segment with the first conductivepath segment. This is due to the fact that current flowing through thevias flows in one direction in the first signal path segment and flowsin an opposite direction in an adjacent signal path segment. Forexample, a mutual inductance formed by the signal path 100 with thesignal path 102 cancels a mutual inductance formed by signal path 102with the signal path 100. Similarly, a mutual inductance formed bysignal path 100 with signal path 98 cancels a mutual inductance formedby signal path 98 with signal path 100.

The physical arrangement of the blind-vias and buried-vias can beconfigured as shown in FIG. 8C for maximizing density of the vias 106the interconnection circuit. FIG. 8B shows blind-vias 106 arranged in arow and column configuration that are parallel to axes of a Cartesiancoordinate system 110, while buried vias 100 are arranged between therows and columns so that the signal paths run at an angle with the axesof Cartesian coordinate system 110, that is, diagonally between theblind-vias and buried-vias. Preferably, the signal paths run at 45° fromthe axes of Cartesian coordinate system 110.

Fiducial Check

The present invention provides a way to check the fiducial registrationof each conductive layer in a laminated structure. Referring to FIG. 9,a kelvin resistor 110 is produced by forming a resistive area 112 at thesame location on each conductive layer of a multilayer substrate in aknown manner using copper or nickel-plated gold, for example, as aresistive material. Leads 114 and 116 are connected to opposite ends ofresistive area 112.

Leads 118 and 120 are connected to the resistive area 112 with apredetermined separation 1_(T). A resistive area 112 is formed at thesame physical location on each conductive layer of the multilayerstructure. Once all layers have been formed, a through-via 122 isdrilled using the laser drilling techniques of the present invention.The through-via 122 is located a predetermined distance 11 from the lead114. The through-via 122 is plated so that all resistive areas 112 areelectrically interconnected.

A known current is injected into resistive area 112 through the lead 114and extracted from lead 116. Leads 118 and 120 are each positioned fromleads 114 and 116 at a point where the current density across resistivearea 112 is uniform and equal. The voltage between leads 114 and 116 ismeasured and the voltage between lead 114 and the through-via 122 ismeasured. The resistance between leads 114 and 116, and between lead 114and the through-via 122 is determined based on the known currentinjected into lead 114 and the dimensions of the resistive area. Theregistration variation 6 of a conductive layer is determined by ##EQU3##where, R₁ is the fractional resistance of resistive area 112 betweenlead 118 and through-via 122;

R₂ is the fractional resistance of resistive area 112 betweenthrough-via 122 and lead 120; and

1_(T) is the predetermined design distance separating leads 118 and 120.

The fractional resistance R₁ is determined by ##EQU4## where, R_(s) isthe resistivity of the material used for resistive area 112,

1₁ is the distance between lead 118 and through-via 122, and

W is the width of resistive area 112.

The fractional resistance R₂ of resistive area 112 is determined by##EQU5## where, 1₂ is the distance between through-via 112 and lead 120.

Flip-Chip Package

One embodiment of a laminated substrate according to the presentinvention is used in fabricating a flip-chip single chip module (SCM)package. FIG. 10 illustrates a flip-chip type chip/package system 124made according to the present invention. System 124 includes a laminatedsubstrate as an interconnect device 126 which is mechanically andelectrically attached to a printed wiring board (PWB) 128 through anarray of solder ball connections 130. The array of solder ballconnections 130 may be a full array or it may be depopulated in the areaunder the semiconductor device 132.

Semiconductor device 132 is mechanically and electrically attached tointerconnect device 126 through an array, full or depopulated, of solderball connections 134. In addition, an underfill adhesive 136 disposedbetween chip 132 and interconnect device 126 reinforces thechip/interconnect device connection. The adhesive is typically a filledepoxy that is dispensed and thermally cured in a well-known manner.

Interconnect device 126 is made of the alternating laminates ofconductive layers and dielectric layers described above, and has acoefficient of thermal expansion (CTE) that nearly matches the CTE ofPWB 128. Additionally, it is required that interconnect device 126 beflat and mechanically robust enough to be easily assembled. Toaccomplish this, a stiffener ring 138 is adhered to interconnect device126. Stiffener ring 138 has a cavity 140 (or cavities) for thesemiconductor device and any other devices, such as capacitors, that areattached on the same surface to interconnect device 126.

In the past, interconnect devices were made of ceramic materials. As thedensity of interconnection on semiconductor devices increases, thesolder ball connections between the interconnect device and the PWB tendto fail due to CTE mismatch. The interconnect device of the presentinvention eliminates this failure mode by closely matching the CTE ofthe interconnect device to the PWB.

The relative thickness of interconnection circuit device 126 of between5 mils to 20 mils, inclusive, causes the device to be adversely affectedby the difference between the CTE of PWB 128 and CTE of the chip 132.Typically, the CTE of a flip-chip PWB varies between 17 to 25 ppm, whilethe CTE of an integrated circuit chip is 3 ppm. The CTE ofinterconnection circuit device 126 is selected so that the differencebetween the CTE α₁ of chip 132 and the CTE α₂ of interconnection circuitdevice 126 is 20 ppm or less. Preferably, the CTE of interconnectioncircuit device 126 is selected to be nominally 18 ppm for minimizingadverse affects caused by the difference between the respective CTEs ofPWB 128 and chip 132. In the region where interconnection circuit device126 is bonded to chip 132, the physical characteristics of chip 132predominate and the effective CTE of interconnection circuit device 126is about 12. In the region where interconnection circuit device 126 isbonded to support ring 138, the characteristics of support ring 138predominate and the effective CTE of interconnection circuit device 126matches the CTE of PWB 128.

The CTE of interconnection circuit device 126 is dominated by thedielectric substrate material forming the device. When interconnectioncircuit device 126 is attached to chip 132, both device 126 and chip 132are heated during solder reflow to typically above 180° C. Dependingupon the dielectric material used for interconnection circuit device126, the dielectric material can be heated above the glass transitiontemperature T_(g) of the dielectric material, causing the CTE of thedielectric material to change by as much as three times the initial CTEof the dielectric material. Consequently, the dielectric material isselected to preferably have a T_(g) to be 200° C. or greater so that theCTE of interconnection circuit device 126 remains nominally 20 ppm/° C.during the temperature range of the solder reflow.

As noted above, interconnect circuit device 126 and chip 132 areadditionally adhesively bonded together using an underfill, typically afilled epoxy, in a known manner. The epoxy underfill shrinks as itcures, thus reducing movement of the chip with respect tointerconnection circuit device 126 so that electrical connections formedby solder balls 134 are less likely to fail. However, the cure shrinkageand bonding at elevated temperature of 150-175° C. of the epoxyunderfill also causes interconnect circuit device 126 and chip 132 tobend, thus separating interconnect circuit device 126 from chip 132 nearthe center of the bond between the parts and stressing the electricalconnections formed by solder balls 134.

To avoid this condition, the integrated circuit chip 132 is selected tobe 25 mils thick or greater. The flexural or bending modulus of a deviceincreases proportionally to the thickness of the device cubed. Thus, byusing an integrated circuit chip that is 25 mils thick or greater, chip132 has less of a tendency to bend in response to a cure shrinkageforce. As a result, the reliability of the packaged semiconductor isenhanced.

Dielectric

Any suitable dielectric material can be used in the present, such as,but not limited to, polyimides and polyimide laminates, epoxy resins,epoxy resins in combination with other resin material, organicmaterials, alone or any of the above combined with fillers. Preferreddielectric materials include a fluoropolymer matrix, where thefluoropolymer can be PTFE, ePTFE or copolymers or blends. Suitablefluoropolymers include, but are not limited to, polytetrafluoroethyleneor expanded polytetrafluoroethylene, with or without an adhesive fillermixture.

Preferred materials include Speedboard® bond plies available from W. L.Gore and Associates, Inc., such as, Speedboard® C which is a prepreg ofnonwoven material containing a cyanate ester resin in apolytetrafluoro-ethylene matrix. Speedboard® C has a dielectricconstant, (Dk) of 2.6-2.7 at 1 MHz-10 GHz, a loss tangent of 0.004 at 1MHz-10 GHz, a dielectric strength greater than 1000 V/mil, a glasstransition (T_(g)) of 220° C., a resin content of 66-68 wt. % and isavailable in a variety of thicknesses. Also Speedboard(® N prepreg,which is a prepreg of a non-woven material containing a multi-functionalepoxy adhesive, in an expanded PTFE matrix may also be used. Speedboard®N has a dielectric constant, (Dk) of 3.0 at 1 MHz, a loss tangent of0.02 at 1 MHz, a dielectric strength greater than 900 V/mil, a glasstransition (T_(g)) of 140° C., a resin content of 66-68 wt. % and isavailable in a variety of thicknesses.

Another suitable dielectric is an expanded PTFE matrix, shown in FIG.11, that includes a mixture of at least two of a cyanate ester compound,an epoxy compound, a bis-triazine compound and a poly (bis-maleimide)resin. For example, a varnish solution is made by mixing 5.95 pounds ofM-30 (Ciba Geigy), 4.07 pounds of RSL 1462 (Shell Resins, Inc.), 4.57pounds of 2,4,6-tribromophenyl-terminated tetrabromobisphenol Acarbonate oligomer (BC-58) (Great Lakes Inc.), 136 g bisphenol A(Aldrich Company), 23.4 g Irganox 1010, 18.1 g of a 10% solution of MnHEX-CEM in mineral spirits, and 8.40 kg MEK. The varnish solution wasfurther diluted into two separate baths--20% (w/w) and 53.8% (w/w). Thetwo varnish solutions were poured into separate impregnation baths, andan e-PTFE web was successively passed through each impregnation bath oneimmediately after the other. The varnish was constantly agitated so asto insure uniformity. The impregnated web was then immediately passedthrough a heated oven to remove all or nearly all the solvent andpartially cure the adhesives, and was collected on a roll. The ePTFE webmay be any desired thickness, such as 25 μm, 40 μm, for example. A 25 μmthick material has a mass of approximately 0.9 g and a weight per areaof approximately 11.2 to 13.8 g/m² Other classes of dielectric materialsinclude those where a porous matrix system contains an imbibed orimpregnated adhesive-filler mixture. The porous matrix is a non-wovensubstrate that is imbibed with high quantities of filler and athermoplastic or thermoset adhesive, as a result of the initial voidvolume of the substrate, heated to partially cure the adhesive and forma B-stage composite. Substrates include fluoropolymers, such as theporous expanded polytetrafluoroethylene material of U.S. Pat. No.3,953,566 to Gore and U.S. Pat. No. 4,482,516 to Bowman, et al., each ofwhich is incorporated herein by reference. Desirably, the mean flow poresize (MFPS) should be between about 2 to 5 times or above that of thelargest particulate, with a MFPS of greater than about 2.4 times that ofthe filler being particularly preferred. However, it is also within thescope of the invention that suitable composites can be prepared byselecting the mean flow particle size MFPS to average particle sizeratio to be greater than 1.4. Acceptable composites can also be preparedwhen the minimum pore size to average particle size is at least above0.8 or the minimum pore size to the maximum particle size is at leastabove 0.4. The MFPS to particle size ratio ratios being performed with amicrotrak FRA analyzer.

Alternatively, another mechanism for gauging relative pore and particlesizes may be calculated as the smallest pore size being not less thanabout 1.4 times the largest particle size.

In addition to expanded fluoropolymer substrates, porous expandedpolyolefins, such as ultra high molecular weight (UHMW) polyethylene,expanded polypropylene, polytetrafluoroethylene made prepared by pasteextrusion and incorporating sacrificial fillers, porous inorganic ororganic foams, microporous cellulose acetate, can also be used.

The porous substrate has an initial void volume of at least 30%,preferably at least 50%, and most preferably at least 70%, andfacilitates the impregnation of thermoset or thermoplastic adhesiveresin and particulate filler paste in the voids while providing aflexible reinforcement to prevent brittleness of the overall compositeand settling of the particles.

The filler comprises a collection of particles when analyzed by aMicrotrak® Model FRA Partical Analyzer device, which displays a maximumparticle size, a minimum particle size and an average particle size byway of a histogram.

Suitable fillers to be incorporated into the adhesive include, but arenot limited to, BaTiO₃, SiO₂, Al₂ O₃, ZnO, ZrO₂, TiO₂, precipitated andsol-gel ceramics, such as silica, titania and alumina, non-conductivecarbon (carbon black) and mixtures thereof. Especially preferred fillersare SiO₂, ZrO₂, TiO₂ alone or incombination with non-conductive carbon.Most preferred fillers include filler made by the vapor metal combustionprocess taught in U.S. Pat. No. 4,705,762, such as, but not limited tosilicon, titanium and aluminum to produced silica, titania, and aluminaparticles that are solid in nature, i.e., not a hollow sphere, with auniform surface curvature and a high degree of sphericity.

The fillers may be treated by well-known techniques that render thefiller hydrophobic by silylating agents and/or agents reactive to theadhesive matrix, such as by using coupling agents. Suitable couplingagents include, silanes, titanates, zirconates, and aluminates. Suitablesilylating agents may include, but are not limited to, functionalsilyating agents, silazanes, silanols, siloxanes. Suitable silazanes,include, but are not limited to, hexamethyldisilazane (Huls H730) andhexamethylcyclotrisilazane, silylamides such as,bis(trimethylsilyl)acetamide (Huls B2500), silylureas such astrimethylsilylurea, and silylmidazoles such as trimethylsilylimidazole.

Titanate coupling agents are exemplified by the tetra alkyl type,monoalkoxy type, coordinate type, chelate type, quaternary salt type,neoalkoxy type, cycloheteroatom type. Preferred titanates include, tetraalkyl titanates, Tyzor® TOT {tetrakis(2-ethyl-hexyl) titanate, Tyzor®TPT {tetraisopropyl titanate}, chelated titanates, Tyzor® GBA {titaniumacetylacetylacetonate}, Tyzor® DC {titanium ethylacetacetonate}, Tyzor®CLA {proprietary to DuPont}, Monoalkoxy (Ken-React® KR TTS), Ken-React®,KR-55 tetra (2,2 diallyloxymethyl)butyl, di(ditridecyl)phosphitotitanate, LICA® 38 neopentyl(diallyl)oxy, tri(dioctyl)pyro-phosphatotitanate.

Suitable zirconates include, any of the zirconates detailed at page 22in the Kenrich catalog, in particular KZ 55-tetra (2,2diallyloxymethyl)butyl, di(ditridecyl)phosphito zirconate,NZ-01-neopentyl(diallyl)oxy, trineodecanoyl zirconate,NZ-09-neopentyl(diallyl)oxy, tri(dodecyl)benzene-sulfonyl zirconate.

The aluminates that can be used in the present invention include, butare not limited to Kenrich(D, diisobutyl(oleyl)acetoacetylaluminate (KA301), diisopropyl(oleyl)acetoacetyl aluminate (KA 322) and KA 489.

In addition to the above, certain polymers, such as, cross-linkedvinylic polymers, e.g., divinylbenzene, divinyl pyridine or a sizing ofany of the disclosed thermosetting matrix adhesives that are firstapplied at very high dilution (0.1 up to 1.0% solution in MEK) can beused. Also, certain organic peroxides, such as, dicumylperoxide can bereacted with the fillers.

The adhesive itself may be a thermoset or thermoplastic and can includepolyglycidyl ether, polycyanurate, polyisocyanate, bis-triazine resins,poly (bis-maleimide), norbornene-terminated polyimide, polynorbornene,acetylene-terminated polyimide, polybutadiene and functionalizedcopolymers thereof, cyclic olefinic polycyclobutene, polysiloxanes, polysisqualoxane, functionalized polyphenylene ether, polyacrylate, novolakpolymers and copolymers, fluoropolymers and copolymers, melaminepolymers and copolymers, poly(bis phenycyclobutane), and blends orprepolymers thereof. It should be understood that the aforementionedadhesives may themselves be blended together or blended with otherpolymers or additives, so as to impact flame retardancy or enhancedtoughness.

As used herein, Mean Flow Pore Size and minimum pore size weredetermined using the Coulter® Porometer II (Coulter Electronics Ltd.,Luton UK) which reports the value directly. Average particle size andlargest particle size were determined using a Microtrac light scatteringparticle size analyzer Model No. FRA (Microtrac Division of Leeds &Northup, North Wales, Pa., USA). The average particle size (APS) isdefined as the value at which 50% of the particles are larger. Thelargest particle size (LPS) is defined as the largest detectableparticle on a Microtrac histogram. Alternatively, the largest particlesize is defined at the minimum point when the Microtrak FRA determinesthat 100% of the particulate have passed.

In general, the method for preparing the adhesive-filler delectircinvolves:(a) expanding a polytetrafluoroethylene sheet by stretching alubricated extruded perform to a microstructure sufficient to allowsmall particles and adhesives to free flow into the void or pore volume;(b) forming a paste from polymeric, e.g., thermoset or thermoplasticmaterial and a filler; and (c) imbibing by dipping, coating, pressurefeeding, the adhesive-filler paste into the highly porous scaffold, suchas expanded polytetrafluoroethylene.

To prepare the filled adhesive film of this invention, particulatefiller is mixed into a solvent or aqueous solution or molten adhesive toafford a finely dispersed mixture. The filler in small particle form isordinarily less than 40 microns in size, and preferably has an averageparticulate size between 1 and 10 microns. The mean pore size of thenode-and-fibril structure of the polytetrafluoroethylene should be largeenough to allow for adequate penetration of the particulate. If thesubstrate is to be expanded polytetrafluoroethylene, then structuressimilar to those taught in U.S. Pat. No. 4,482,516 to Bowman, et al. aredesirable. Desirably, the mean flow pore size (MFPS) should be betweenabout 2 to 5 times or above that of the largest particulate, with a MFPSof greater than about 2.4 times that of the filler being particularlypreferred. However, it is also within the scope of the invention thatsuitable composites can be prepared by selecting the mean flow particlesize MFPS to average particle size ratio to be greater than 1.4.Acceptable composites can also be prepared when the minimum pore size toaverage particle size is at least above 0.8 or the minimum pore size tothe maximum particle size is at least above 0.4. The MFPS to particlesize ratio ratios being performed with a microtrak FRA analyzer.

Alternatively, another mechanism for gauging relative pore and particlesizes may be calculated as the smallest pore size being not less thanabout 1.4 times the largest particle size.

Table 17 shows the effect of the relationship of the substrate mean flowpore size (MFPS) and particulate size. When the ratio of the mean flowpore size (MFPS) to largest particulate is 1.4 or less, poor results areobserved. In this case, a homogeneous composite is not observed, andmost of the particulate filler does not uniformly penetrate themicroporous substrate. When the ratio of the MFPS to largest particulateis greater than about 2.0, then a uniform composite is obtained. It isalso observed that the larger the ratio of MFPS to largest particulate,the greater the relative case it is to imbibe a homogeneous dispersioninto the microporous substrate.

                                      TABLE 17                                    __________________________________________________________________________    Substrate   Particle                                                            Pore Size Size MFPS Pore.sub.Min                                                Min MFPS                                                                              Avg Max ÷                                                                              ÷                                                                             Pore.sub.Min ÷                                 Sample (μm) (μm) (μm) (μm) Part.sub.Avg Part.sub.Max                                               Part.sub.Avg Result                        __________________________________________________________________________    A   4   7   5   10  1.4  0.4 0.8   Poor                                         B 4 5 5 10 1.0 0.4 0.8 Poor                                                   C -- 58 5 10 12.4 N/A -- Good                                                 D 18 32 6 10 5.3 1.8 3.0 Good                                                 E 18 32 1 1 32.0 18.0 18 Good                                                 F 17 24 6 10 4.0 1.7 2.8 Good                                                 G 0.2 0.4 0.5 1.6 0.8 0.125 0.4 Poor                                          H -- 60 18 30 3.3 -- -- Good                                                  I 14 11 0.5 1.6 22.0 8.8 28 Good                                              J 14 29 4 8 7.3 1.8 3.5 Good                                                  K 14 29 5 10 5.8 1.4 2.8 Good                                               __________________________________________________________________________

EXAMPLE 1

A fine dispersion was prepared by mixing 281.6 g TiO₂ (TI Pure R-900, DuPont Company) into a 20% (w/w) solution of a flame retardeddicyanamide/2-methylimidazole catalyzed bisphenol-A based polyglycidylether (Nelco N-4002-5, Nelco Corp.) in MEK. The dispersion wasconstantly agitated so as to insure uniformity. A swatch of expandedPTFE was then dipped into the resin mixture. The web was dried at 165°C. for 1 min. under tension to afford a flexible composite. Thepartially-cured adhesive composite thus produced comprised of 57 weightpercent TiO₂, 13 weight percent PTFE and 30 weight percent epoxyadhesive. Several plies of the adhesive sheet were laid up betweencopper foil and pressed at 600 psi in a vacuum-assisted hydraulic pressat temperature of 225° C. for 90 min. then cooled under pressure. Thisresulted in a copper laminate having dielectric constant of 19.0, andwithstood a 30 sec. solder shock at 280° C. at an average ply thicknessof 100 mm (0.0039"(3.9 mil)) dielectric laminate thickness.

EXAMPLE 2

A fine dispersion was prepared by mixing 386 g SiO₂ (HW-11-89, HarbisonWalker Corp.) which was pretreated with phenyltrimethoxysilane (04330,Huls/Petrarch) into a manganese catalyzed solution of 200 g bismaleimidetriazine resin (BT206OBJ, Mitsubishi Gas Chemical) and 388 g MEK. Thedispersion was constantly agitated so as to insure uniformity. A swatchof 0.0002" thick expanded PTFE was then dipped into the resin mixture,removed, and then dried at 165° C. for 1 min. under tension to afford aflexible composite. Several plies of this prepreg were laid up betweencopper foil and pressed at 250 psi in a vacuum-assisted hydraulic pressat temperature of 225° C. for 90 min. then cooled under pressure. Thisresulting dielectric thus produced comprised of 53 weight percent SiO₂,5 weight percent PTFE and 42 weight percent adhesive, displayed goodadhesion to copper, dielectric constant (at 10 GHz) of 3.3 anddissipation factor (at 10 GHz) of 0.005.

EXAMPLE 3

A fine dispersion was prepared by mixing 483 g SiO₂ (HW-11-89) into amanganese-catalyzed solution of 274.7 g bismaleimide triazine resin(BT2060BJ, Mitsubishi Gas Chemical) and 485 g MEK. The dispersion wasconstantly agitated so as to insure uniformity. A swatch of 0.0002"thick expanded PTFE was then dipped into the resin mixture, removed, andthen dried at 165° C. for 1 min. under tension to afford a flexiblecomposite. Several plies of this prepreg were laid up between copperfoil and pressed at 250 psi in a vacuum-assisted hydraulic press attemperature of 225° C. for 90 minutes then cooled under pressure. Theresulting dielectric thus produced comprised of 57 weight percent SiO₂,4 weight percent PTFE and 39 weight percent adhesive, displayed goodadhesion to copper, dielectric constant (at 10 GHz) of 3.2 anddissipation factor (at 10 GHz) of 0.005.

EXAMPLE 4

A fine dispersion was prepared by mixing 15.44 kg TiO₂ powder (TI PureR-900, DuPont Company) into a manganese-catalyzed solution of 3.30 kgbismaleimide triazine resin (BT206OBH, Mitsubishi Gas Chemical) and15.38 kg MEK. The dispersion was constantly agitated so as to insureuniformity. A swatch of 0.0004" TiO₂ -filled expanded PTFE (filledaccording to the teachings of Mortimer U.S. Pat. No. 4,985,296, exceptto 40% loading of TiO₂ and the membrane was not compressed at the end)was then dipped into the resin mixture, removed, and then dried at 165°C. for 1 min. under tension to afford a flexible composite. Thepartially cured adhesive composite thus produced comprised of 70 weightpercent TiO₂, 9 weight percent PTFE and 21 weight percent adhesive.Several plies of this prepreg were laid up between copper foil andpressed at 500 psi in a vacuum-assisted hydraulic press at temperatureof 220° C. for 90 minutes then cooled under pressure. This resultingdielectric displayed good adhesion to copper, dielectric constant of10.0 and dissipation factor of 0.008.

EXAMPLE 5

A fine dispersion was prepared by mixing 7.35 kg SiO₂ (ADMATECHS SO-E2,Tatsumori LTD) with 7.35 kg MEK and 73.5 g of coupling agent, i.e.,3-glycidyloxypropyltri-methoxysilane (Dynasylan GLYMO (Petrach Systems).SO-E2 is described by the manufacture as having highly spherical silicahaving a particle diameter of 0.4 to 0.6 mm, a specific surface area of4-8 m² /g, a bulk density of 0.2-0.4 g/cc (loose).

To this dispersion was added 932 g of a 50% (w/w) solution of a cyanatedphenolic resin, Primaset PT-30 (Lonza Corp.). In (MEK)methylethylketone, 896 g of a 50% (w/w) solution of RSL 1462 (ShellResins, Inc.(CAS #25068-38-6)) in MEK, 380 g of a 50% (w/w) solution ofBC-58 (Great Lakes, Inc.) in MEK, 54 g of 50% solution of bisphenol A(Aldrich Company) in MEK, 12.6 g Irganox 1010 (Ciba Geigy), 3.1 g of a0.6% solution of Manganese 2-ethylhexanoate (Mn HEX-CEM (OMG Ltd.), and2.40 kg MEK. This dispersion was subjected to ultrasonic agitationthrough a Misonics continuous flow cell for about 20 minutes at a rateof about 1-3 gal./minute. The fine dispersion thus obtained was furtherdiluted to an overall bath concentration of 11.9% solids (w/w).

The fine dispersion was poured into an impregnation bath. A expandedpolytetrafluoroethylene web having the node fibril structure of FIG. 12,and the following properties:

    ______________________________________                                        Frazier               20.55                                                     Coverage 9 g/m.sup.2                                                          Ball Burst 3.2 lbs.                                                           Thickness 6.5 mil.                                                            Mean Flow Pore Size 9.0 microns                                             ______________________________________                                    

The Frazier number relates to the air permeability of the material beingassayed. Air permeability is measured by clamping the web in a gasketedfixture which is provided in circular area of approximately 6 squareinches for air flow measurement. The upstream side was connected to aflow meter in line with a source of dry compressed air. The downstreamside of the sample fixture was open to the atmosphere. Testing isaccomplished by applying a pressure of 0.5 inches of water to theupstream side of the sample and recording the flow rate of the airpassing through the in-line flowmeter (a ball-float rotameter that wasconnected to a flow meter.

The Ball Burst Strength is a test that measures the relative strength ofsamples by determining the maximum at break. The web is challenged witha 1 inch diameter ball while being clamped between two plates. TheChatillon, Force Gauge Ball/Burst Test was used. The media is plaacedtaut in the measuring device and pressure afixed by raising the web intocontact with the ball of the burst probe. Pressure at break is recorded.

The web described above was passed through a constantly agitatedimpregnation bath at a speed at or about 3 ft./min, so as to insureuniformity. The impregnated web is immediately passed through a heatedoven to remove all or nearly all the solvent, and is collected on aroll.

Several plies of this prepeg were laid up between copper foil andpressed at 200 psi in a vacuum-assisted hydraulic press at temperatureof 220° C. for 90 minutes and then cooled under pressure. This resultingdielectric displayed good adhesion to copper, dielectric constant (10GHz) of 3.0 and dissipation factor of 0.0085 (10 GHz).

The physical properties of the particulate filler used in Example 4 andExample 7 are compared below.

    ______________________________________                                                      Tatsumori                                                         Property (ADMATECHS) Harbison Walker                                        ______________________________________                                        Manufacture Technique                                                                       Vapor Metal Amorphous Fused Silica                                 Combustion                                                                   Designation Silica SO-E2 HW-11-89                                             Median Particle Size 0.5 micron 5 micron                                      Shape Spherical Irregular, jagged                                             Surface Area 6-10 m.sup.2 /g 10 m.sup.2 /g                                    Bulk Density 0.47 g/cc 1.12 g/cc                                              Specific Density 2.26 g/cc 2.16 g/cc                                        ______________________________________                                    

EXAMPLE 6

An ePTFE matrix containing an impregnated adhesive filler mixture, basedon SiO₂ prepared from the vapor combustion of molten silicon is preparedas follows. Two precursor mixtures were initially prepared. One being inthe form of a slurry containing a silane treated silica similar to thatof Example 5 and the other an uncatalyzed blend of the resin and othercomponents.

Mixture I

The silica slurry is a 50/50 blend of the SO-E2 silica of Example 5 inMEK, where the silica contains a coated of silane which is equal to 1%of the silica weight. To a five gallon container, 17.5 pounds of MEK and79 grams of silane were added and the two components mixed to ensureuniform dispersion of the silane in the MEK. Then, 17.5 pounds of thesilica of Example 5 were added. Two five gallon containers of theMEK-silica-silane mixture were added to a reaction vessel, and thecontents, i.e., the slurry, was recirculated through an ultrasonicdisperser for approximately one hour to break up any silica agglomeratesthat may be present. The sonication was completed and the contents ofthe reaction vessel were heated to approximately 80° C. forapproximately one hour, while the contents were continuously mixed. Thereacted mixture was then transferred into a ten gallon container.

Mixture II

The desired resin blend product is an MEK based mixture containing anuncatalyzed resin blend (the adhesive) contains approximately 60%solids, where the solid portion is an exact mixture of 41.2% PT-30cyanated phenolic resin, 39.5% RSL 1462 epoxy resin, 16.7% BC58 flameretardant, 1.5% Irganox 1010 stabilizer, and 1% bisphenol A co-catalyst,all percentages by weight.

Into a ten gallon container, 14.8 pounds of PT-30 and 15-20 pounds ofMEK were added and stirred vigorously to completely solvate the PT-30.Then 6 pounds of BC58 were measured and added to the MEK/PT-30 solutionand vigorously agitated to solvate the BC58. The stabilizer, 244.5 gramsof Irganox 1010 and bisphenol A, 163 grams were added. The ten galloncontainer was reweighed and 14.22 pounds of RSL 1462 were added.Additional MEK was added to bring the mixture weight to 60 pounds. Thecontents were then vigorously agitated for approximately 1 to 2 hours,or as long is necessary to completely dissolve the solid components.

The desired product is a mixture of the silica treated with a silane,the uncatalyzed resin blend, and MEK in which 68% by weight of thesolids are silica, and the total solids are between 5% and 50% by weightof the mixture. The exact solids concentration varies from run to run,and depends in part on the membrane to be impregnated. The catalystlevel is 10 ppm relative to the sum of the PT-30 and RSL1462.

The solid contents of mixtures I and II were determined to verify theaccuracy of the precursors and compensate for any solvent flash that hadoccurred. Then mixture I was added to a ten gallon container to provide12 pounds of solids, e.g., 515 solids content, 23.48 pounds ofmixture 1. Then mixture II was added to the container to provide 5.64pounds of solids, e.g., 59.6% solids, 9.46 pounds of mixture II. themanganese catalyst solution (0.6% in mineral spirits), 3.45 grams, wasadded to the mixture of mixture I and mixture II and blended thoroughlyto form a high solids content mixture.

The bath mixture for impregnating an ePTFE matrix, 28% solids mixture,was prepared by adding sufficient MEK to the high solids content mixtureto a total weight of 63 pounds.

Thereafter, an ePTFE matrix weas impregnated with this bath mixture toform a dielectric material.

EXAMPLE 7

A fine dispersion was prepared by mixing 26.8 grams Furnace Black(Special Schwarz 100, Degussa Corp., Ridgefield Park, N.J.) and 79 gramsof coupling agent (Dynaslan GLYMO CAS #2530-83-8;3-glycidyloxypropyl-trimethoxysilane (Petrach Systems). The dispersionwas subjected to ultrasonic agitation for 1 minute, then added to astirring dispersion of 17.5 pounds SiO₂ (SO-E2) in 17.5 pounds MEK whichhad previously been ultrasonically agitated. The final dispersion washeated with constant overhead mixing for 1 hour at reflux, then allowedto cool to room temperature.

Separately, an adhesive varnish was prepared by adding the following:3413 grams of a 57.5% (w/w) mixture of Primaset PT-30 in MEK, 2456 gramsof a 76.8% (w/w/) mixture of RSL 1462 in MEK, 1495 grams of a 53.2%(w/w) solution of BC58 (Great Lakes, Inc.) in MEK, 200 grams of 23.9%(w/w) solution of bisphenol A (Aldrich Company) in MEK, 71.5 gramsIrganox 1010, 3.21 grams of a 0.6% (w/w) solution of Mu HEX-CEM (OMGLtd.) in mineral spirits, and 2.40 kg MEK.

In a separate container, 3739 grams of the dispersion described abovewas added, along with 0.0233 grams of Furnace Black (Special Schwarz100, Degussa Corp., Ridgefield Park, N.J.), 1328 of the adhesive varnishdescribed above and 38.3 pounds MEK. This mixture was poured into animpregnation bath, and an ePTFE web was passed through the impregnationbath at a speed at or about 3 ft/min. This dispersion was constantlyagitated so as to insure uniformity. The impregnated web is immediatelypassed through a heated oven to remove all or nearly all the solvent,and is collected on a roll.

Several piles of this prepeg were laid up between copper foil andpressed at 200 psi in a vacuum-assisted hydraulic press at temperaturesof 200° C. for 90 minutes then cooled under pressure. This resultingdielectric displayed good adhesion to copper.

EXAMPLE 8

An adhesive varnish was prepared by adding the following: 3413 grams ofa 57.5% (w/w) solution of Primaset PT-30 (PMN P-88-1591)) in MEK, 2456grams of a 76.8% (w/w) solution of RSL 1462 in MEK, 1495 grams of a53.2% (w/w) solution of BC58 (Great Lakes, Inc.) in MEK, 200 grams of23.9% (w/w) solution of bisphenol A (Aldrich Company) in MEK, 71.5 gramsIrganox 1010, 3.21 grams of a 0.6% (w/w) solution of Mn HEX-CEM inmineral spirits, and 2.40 kg MEK.

In a separate container, 1328 grams of the adhesive varnish describedabove, 42.3 pounds MEK, 6.40 grams of Furnace Black (Special Schwarz100, Degussa Corp., Ridgefield, N.J. and 1860.9 grams SiO₂ (SO-E2). Thismixture was poured into an impregnation bath, and an ePTFE web waspassed through the impregnation bath at a speed at or about 3 ft/min.The dispersion was constantly agitated so as to insure uniformity. Theimpregnated web is immediately passed through a heated oven to removeall or nearly all the solvent, and is collected on a roll.

Several piles of this prepeg were laid up between copper foil andpressed at 200 psi in a vacuum-assisted hydraulic press at temperatureof 220° C. for 90 minutes then cooled under pressure. This resultingdielectric displayed good adhesion to copper.

While the present invention has been described in connection with theillustrated embodiments, it will be appreciated and understood thatmodifications may be made without departing from the true spirit andscope of the invention.

What is claimed is:
 1. A method for forming a through-via in a laminatedsubstrate, comprising the steps of:laser drilling the through-via in alaminated substrate from a top exposed surface of the substrate to abottom exposed surface of the substrate using a plurality of laserpulses that are trepanned in a first predetermined pattern, each pulsetrepanned in the first predetermined pattern having a first energydensity per pulse; and laser drilling the through-via using a pluralityof laser pulses that are trepanned in a second predetermined pattern,each pulse trepanned in the second predetermined pattern having a secondenergy density per pulse, the second energy density per pulse beinggreater than the first energy density per pulse, the secondpredetermined pattern being within the first predetermined pattern. 2.The method according to claim 1, wherein an entrance width of thethrough-via is ≦75 μm.
 3. The method according to claim 2, wherein anaspect ratio of the through-via is ≧3:1.
 4. The method according toclaim 3, wherein the steps of claim 1 are performed in situ.
 5. Themethod according to claim 4, wherein when the aspect ratio of thethrough-via is 10:1, a variance of the exit width of the through-via isabout 10 μm².
 6. The method according to claim 4, wherein when theaspect ratio of the through-via is 20:1, a variance of the exit width ofthe through-via is about 15 μm².
 7. The method according to claim 4,wherein the first predetermined pattern is a circle having a firstdiameter, and the second predetermined pattern is circle having a seconddiameter, the first diameter being greater than the second diameter. 8.The method according to claim 4, further comprising the stepsof:applying a polymeric photo-absorptive layer on the exposed topsurface of the substrate before laser drilling the substrate; formingablated material by laser drilling the through-via in the substratethrough the photo-absorptive layer; redepositing the ablated material onthe photo-absorptive layer surrounding the aperture; and enhancing anentrance of the through-via by removing the photo-absorptive layer andthe ablated redeposited material on the photo-absorptive layer.
 9. Themethod according to claim 8, wherein the substrate includes a conductivelayer, andwherein the first energy density per pulse is greater than anablation energy density threshold of the conductive layer.
 10. Themethod according to claim 9, wherein the laser emits at a wavelength of355 nm, andwherein the first energy density per pulse is greater than 5J/cm².
 11. The method according to claim 10, wherein the first energydensity per pulse applied to the substrate is nominally 11 J/cm². 12.The method according to claim 11, wherein each pulse has a pulsewidth≦100 ns.
 13. The method according to claim 9, wherein the laseremits at a wavelength of 266 nm, and wherein the first energy densityper pulse is greater than 1.5 J/cm².
 14. The method according to claim13, wherein the first energy density per pulse applied to the substrateis nominally 5 J/cm².
 15. The method according to claim 14, wherein eachpulse has a pulse width≦100 ns.
 16. The method according to claim 8,wherein the substrate includes a conductive layer, andwherein the secondenergy density per pulse is greater than an ablation energy densitythreshold of the conductive layer.
 17. The method according to claim 16,wherein the laser emits at a wavelength of 355 nm, andwherein the secondenergy density per pulse is greater than 5 J/cm².
 18. The methodaccording to claim 17, wherein the second energy density per pulseapplied to the substrate is nominally 11 J/cm².
 19. The method accordingto claim 18, wherein each pulse has a pulse width≦100 ns.
 20. The methodaccording to claim 16, wherein the laser emits at a wavelength of 266nm, and wherein the second energy density per pulse is greater than 1.5J/cm².
 21. The method according to claim 20, wherein the second energydensity per pulse applied to the substrate is nominally 5 J/cm².
 22. Themethod according to claim 21, wherein each pulse has a pulse width≦100ns.
 23. The method according to claim 8, further comprising the step offorming a plurality of through-vias by performing the step of laserdrilling the through-via in the substrate from the top surface of thesubstrate to the bottom surface of the substrate using a plurality oflaser pulses that are trepanned in a first predetermined pattern and thestep of laser drilling the through-via using a plurality of laser pulsesthat are trepanned in a second predetermined pattern in situ at aplurality of locations in the substrate.
 24. The method according toclaim 23, further comprising the step of laser drilling at least oneblind-via in the substrate using a plurality of laser pulses for eachblind-via.
 25. The method according to claim 24, wherein the substrateincludes a conductive layer formed on a dielectric layer, the conductivelayer being the top exposed layer of the substrate, the conductive layerhaving a performed aperture corresponding to a location of eachblind-via in the substrate, each aperture exposing a surface of thedielectric layer, andwherein the step of forming the blind-vias in thesubstrate by laser drilling includes the step of applying an energydensity per pulse to the substrate layer for each blind-via that isgreater than an ablation energy density threshold of the dielectriclayer and less than an ablation energy density threshold of theconductive layer.
 26. The method according to claim 25, wherein anentrance width of each blind-via is ≦75 μm.
 27. The method according toclaim 26, wherein at least one blind-via has an aspect ratio of 1:1 orgreater.
 28. The method according to claim 27, wherein the laser emitsat a wavelength of 355 nm; andwherein the energy density per pulseapplied to the substrate for laser drilling each blind-via is between0.5 J/cm² and 11 J/cm² inclusive.
 29. The method according to claim 28,wherein the energy density per pulse applied to the substrate for laserdrilling each blind-via is nominally 5 J/cm².
 30. The method accordingto claim 29, wherein each pulse has a pulse width≦100 ns.
 31. The methodaccording to claim 27, wherein the laser emits at a wavelength of 266nm; andwherein the energy density per pulse applied to the substrate fordrilling each blind-via is between 0.5 J/cm² and 3 J/cm² inclusive. 32.The method according to claim 31, wherein the energy density per pulseapplied to the substrate for drilling each blind-via is nominally 2J/cm².
 33. The method according to claim 32, wherein each pulse has apulse width≦100 ns.